diff --git a/Cargo.lock b/Cargo.lock
index 8ff6fc3ef..20ae00caf 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2229,9 +2229,11 @@ dependencies = [
  "js-sys",
  "lightcraft-codecs",
  "lightcraft-engine",
+ "lightcraft-preview",
  "lightcraft-raster",
  "lightcraft-ui-egui",
  "log",
+ "serde",
  "serde_json",
  "wasm-bindgen",
  "wasm-bindgen-futures",
diff --git a/apps/lightcraft-web/Cargo.toml b/apps/lightcraft-web/Cargo.toml
index b9c7bdf23..1f9f14c13 100644
--- a/apps/lightcraft-web/Cargo.toml
+++ b/apps/lightcraft-web/Cargo.toml
@@ -17,6 +17,8 @@ lightcraft-engine = { workspace = true }
 lightcraft-ui-egui = { workspace = true }
 lightcraft-codecs = { workspace = true }
 lightcraft-raster = { workspace = true }
+lightcraft-preview = { workspace = true }
+serde = { workspace = true }
 serde_json = { workspace = true }
 log = { workspace = true }
 
@@ -29,16 +31,42 @@ js-sys = { workspace = true }
 web-sys = { workspace = true, features = [
     "Blob",
     "BlobPropertyBag",
+    "DedicatedWorkerGlobalScope",
     "Document",
+    "DomStringList",
     "Element",
+    "EventTarget",
     "File",
     "FileList",
+    "FileSystemCreateWritableOptions",
+    "FileSystemDirectoryHandle",
+    "FileSystemFileHandle",
+    "FileSystemGetDirectoryOptions",
+    "FileSystemGetFileOptions",
+    "FileSystemRemoveOptions",
+    "FileSystemWritableFileStream",
     "HtmlAnchorElement",
     "HtmlCanvasElement",
     "HtmlElement",
     "HtmlInputElement",
+    "IdbDatabase",
+    "IdbFactory",
+    "IdbKeyRange",
+    "IdbObjectStore",
+    "IdbOpenDbRequest",
+    "IdbRequest",
+    "IdbTransaction",
+    "IdbTransactionMode",
+    "IdbVersionChangeEvent",
     "Location",
+    "MessageEvent",
+    "Navigator",
     "Performance",
+    "StorageManager",
     "Url",
     "Window",
+    "Worker",
+    "WorkerOptions",
+    "WorkerType",
+    "WritableStream",
 ] }
diff --git a/apps/lightcraft-web/index.html b/apps/lightcraft-web/index.html
index ac1ed167a..bc0ad2037 100644
--- a/apps/lightcraft-web/index.html
+++ b/apps/lightcraft-web/index.html
@@ -22,8 +22,16 @@
   <div id="lightcraft_loading">Loading LightCraft…</div>
   <div id="lightcraft_status" aria-live="polite"></div>
   <script type="module">
-    import init from "./lightcraft_web.js";
-    init().catch((e) => {
+    import init, { start, command } from "./lightcraft_web.js";
+    (async () => {
+      // Compile once; the render workers (worker.js) instantiate the same compiled module.
+      const module = await WebAssembly.compileStreaming(fetch("./lightcraft_web_bg.wasm"));
+      globalThis.lightcraftModule = module;
+      await init({ module_or_path: module });
+      // automation: `await lightcraft.command("library.info", "{}")` → JSON text
+      globalThis.lightcraft = { command };
+      start();
+    })().catch((e) => {
       console.error(e);
       const el = document.getElementById("lightcraft_loading");
       if (el) el.textContent = "LightCraft failed to load: " + e;
diff --git a/apps/lightcraft-web/src/backend.rs b/apps/lightcraft-web/src/backend.rs
new file mode 100644
index 000000000..50a03135f
--- /dev/null
+++ b/apps/lightcraft-web/src/backend.rs
@@ -0,0 +1,324 @@
+//! Browser storage, asynchronous: OPFS (Origin Private File System) when the page can write it
+//! from the main thread (`FileSystemFileHandle.createWritable`), else IndexedDB. Used from the
+//! window and from render workers alike. Keys are `/`-separated paths (OPFS directories; plain
+//! IndexedDB keys).
+
+use js_sys::{Reflect, Uint8Array};
+use wasm_bindgen::prelude::*;
+use wasm_bindgen::{JsCast, JsValue};
+use wasm_bindgen_futures::JsFuture;
+use web_sys::{FileSystemDirectoryHandle, FileSystemFileHandle, FileSystemGetDirectoryOptions, FileSystemGetFileOptions, IdbDatabase};
+
+const IDB_NAME: &str = "lightcraft";
+const IDB_STORE: &str = "files";
+
+#[derive(Clone)]
+pub enum Backend {
+    Opfs(FileSystemDirectoryHandle),
+    Idb(IdbDatabase),
+}
+
+fn err(e: JsValue) -> String {
+    e.as_string().or_else(|| e.dyn_ref::<js_sys::Error>().map(|e| String::from(e.message()))).unwrap_or_else(|| format!("{e:?}"))
+}
+
+/// `navigator.storage` in a window or a worker.
+fn storage_manager() -> Option<web_sys::StorageManager> {
+    let g = js_sys::global();
+    let nav = Reflect::get(&g, &"navigator".into()).ok()?;
+    let storage = Reflect::get(&nav, &"storage".into()).ok()?;
+    (!storage.is_undefined()).then(|| storage.unchecked_into())
+}
+
+fn indexed_db() -> Option<web_sys::IdbFactory> {
+    let f = Reflect::get(&js_sys::global(), &"indexedDB".into()).ok()?;
+    (!f.is_undefined() && !f.is_null()).then(|| f.unchecked_into())
+}
+
+/// Can files be written through OPFS writable streams here?
+fn opfs_writable() -> bool {
+    let g = js_sys::global();
+    Reflect::get(&g, &"FileSystemFileHandle".into())
+        .ok()
+        .filter(|c| !c.is_undefined())
+        .and_then(|c| Reflect::get(&c, &"prototype".into()).ok())
+        .and_then(|p| Reflect::get(&p, &"createWritable".into()).ok())
+        .is_some_and(|f| f.is_function())
+}
+
+/// Resolve an IndexedDB request.
+async fn idb_req(req: &web_sys::IdbRequest) -> Result<JsValue, String> {
+    let r1 = req.clone();
+    let p = js_sys::Promise::new(&mut |resolve, reject| {
+        let r2 = r1.clone();
+        let ok = Closure::once_into_js(move || {
+            let _ = resolve.call1(&JsValue::NULL, &r2.result().unwrap_or(JsValue::UNDEFINED));
+        });
+        let fail = Closure::once_into_js(move |e: JsValue| {
+            let _ = reject.call1(&JsValue::NULL, &e);
+        });
+        r1.set_onsuccess(Some(ok.unchecked_ref()));
+        r1.set_onerror(Some(fail.unchecked_ref()));
+    });
+    JsFuture::from(p).await.map_err(err)
+}
+
+/// Wait for an IndexedDB transaction to commit.
+async fn idb_done(tx: &web_sys::IdbTransaction) -> Result<(), String> {
+    let t1 = tx.clone();
+    let p = js_sys::Promise::new(&mut |resolve, reject| {
+        let ok = Closure::once_into_js(move || {
+            let _ = resolve.call0(&JsValue::NULL);
+        });
+        let rej = reject.clone();
+        let fail = Closure::once_into_js(move |e: JsValue| {
+            let _ = rej.call1(&JsValue::NULL, &e);
+        });
+        let abort = Closure::once_into_js(move |e: JsValue| {
+            let _ = reject.call1(&JsValue::NULL, &e);
+        });
+        t1.set_oncomplete(Some(ok.unchecked_ref()));
+        t1.set_onerror(Some(fail.unchecked_ref()));
+        t1.set_onabort(Some(abort.unchecked_ref()));
+    });
+    JsFuture::from(p).await.map(|_| ()).map_err(err)
+}
+
+impl Backend {
+    /// OPFS if writable from here (unless `prefer_idb`), else IndexedDB.
+    pub async fn open(prefer_idb: bool) -> Result<Backend, String> {
+        if !prefer_idb
+            && opfs_writable()
+            && let Some(sm) = storage_manager()
+        {
+            match JsFuture::from(sm.get_directory()).await {
+                Ok(root) => return Ok(Backend::Opfs(root.unchecked_into())),
+                Err(e) => log::warn!("OPFS unavailable ({}); using IndexedDB", err(e)),
+            }
+        }
+        Self::open_idb().await
+    }
+
+    async fn open_idb() -> Result<Backend, String> {
+        let f = indexed_db().ok_or("no IndexedDB")?;
+        let req = f.open_with_u32(IDB_NAME, 1).map_err(err)?;
+        let upgrade = Closure::<dyn FnMut(web_sys::IdbVersionChangeEvent)>::new(move |e: web_sys::IdbVersionChangeEvent| {
+            if let Some(db) = e.target().and_then(|t| t.dyn_into::<web_sys::IdbOpenDbRequest>().ok()).and_then(|r| r.result().ok()) {
+                let db: IdbDatabase = db.unchecked_into();
+                if !db.object_store_names().contains(IDB_STORE) {
+                    let _ = db.create_object_store(IDB_STORE);
+                }
+            }
+        });
+        req.set_onupgradeneeded(Some(upgrade.as_ref().unchecked_ref()));
+        let db = idb_req(&req).await?;
+        drop(upgrade);
+        Ok(Backend::Idb(db.unchecked_into()))
+    }
+
+    /// "opfs" or "idb" (workers open the same kind).
+    pub fn kind(&self) -> &'static str {
+        match self {
+            Backend::Opfs(_) => "opfs",
+            Backend::Idb(_) => "idb",
+        }
+    }
+
+    /// The directory holding `path`'s last component (created when `create`).
+    async fn parent(root: &FileSystemDirectoryHandle, path: &str, create: bool) -> Result<Option<(FileSystemDirectoryHandle, String)>, String> {
+        let mut parts: Vec<&str> = path.split('/').filter(|p| !p.is_empty()).collect();
+        let name = parts.pop().ok_or("empty path")?.to_string();
+        let mut dir = root.clone();
+        for p in parts {
+            let opts = FileSystemGetDirectoryOptions::new();
+            opts.set_create(create);
+            match JsFuture::from(dir.get_directory_handle_with_options(p, &opts)).await {
+                Ok(d) => dir = d.unchecked_into(),
+                Err(e) if !create => {
+                    let _ = e;
+                    return Ok(None);
+                }
+                Err(e) => return Err(err(e)),
+            }
+        }
+        Ok(Some((dir, name)))
+    }
+
+    async fn opfs_file(root: &FileSystemDirectoryHandle, path: &str, create: bool) -> Result<Option<FileSystemFileHandle>, String> {
+        let Some((dir, name)) = Self::parent(root, path, create).await? else { return Ok(None) };
+        let opts = FileSystemGetFileOptions::new();
+        opts.set_create(create);
+        match JsFuture::from(dir.get_file_handle_with_options(&name, &opts)).await {
+            Ok(h) => Ok(Some(h.unchecked_into())),
+            Err(_) if !create => Ok(None),
+            Err(e) => Err(err(e)),
+        }
+    }
+
+    fn idb_store(db: &IdbDatabase, write: bool) -> Result<(web_sys::IdbTransaction, web_sys::IdbObjectStore), String> {
+        let mode = if write { web_sys::IdbTransactionMode::Readwrite } else { web_sys::IdbTransactionMode::Readonly };
+        let tx = db.transaction_with_str_and_mode(IDB_STORE, mode).map_err(err)?;
+        let store = tx.object_store(IDB_STORE).map_err(err)?;
+        Ok((tx, store))
+    }
+
+    /// A whole file, or `None` if it doesn't exist.
+    pub async fn read(&self, path: &str) -> Result<Option<Vec<u8>>, String> {
+        match self {
+            Backend::Opfs(root) => {
+                let Some(h) = Self::opfs_file(root, path, false).await? else { return Ok(None) };
+                let file: web_sys::File = JsFuture::from(h.get_file()).await.map_err(err)?.unchecked_into();
+                let buf = JsFuture::from(file.array_buffer()).await.map_err(err)?;
+                Ok(Some(Uint8Array::new(&buf).to_vec()))
+            }
+            Backend::Idb(db) => {
+                let (_tx, store) = Self::idb_store(db, false)?;
+                let v = idb_req(&store.get(&path.into()).map_err(err)?).await?;
+                if v.is_undefined() || v.is_null() {
+                    return Ok(None);
+                }
+                Ok(Some(Uint8Array::new(&v).to_vec()))
+            }
+        }
+    }
+
+    /// Replace a file atomically (OPFS: the writable's swap file is committed on close).
+    pub async fn write(&self, path: &str, data: &[u8]) -> Result<(), String> {
+        self.write_at(path, None, data).await
+    }
+
+    /// Write `data` at `offset` keeping the bytes before it (`None`: replace the file). Atomic.
+    pub async fn write_at(&self, path: &str, offset: Option<usize>, data: &[u8]) -> Result<(), String> {
+        match self {
+            Backend::Opfs(root) => {
+                let h = Self::opfs_file(root, path, true).await?.ok_or("can't create file")?;
+                let opts = web_sys::FileSystemCreateWritableOptions::new();
+                opts.set_keep_existing_data(offset.is_some());
+                let w: web_sys::FileSystemWritableFileStream =
+                    JsFuture::from(h.create_writable_with_options(&opts)).await.map_err(err)?.unchecked_into();
+                let res = async {
+                    if let Some(off) = offset {
+                        JsFuture::from(w.truncate_with_f64(off as f64).map_err(err)?).await.map_err(err)?;
+                        JsFuture::from(w.seek_with_f64(off as f64).map_err(err)?).await.map_err(err)?;
+                    }
+                    JsFuture::from(w.write_with_js_u8_array(&Uint8Array::from(data)).map_err(err)?).await.map_err(err)?;
+                    Ok::<(), String>(())
+                }
+                .await;
+                match res {
+                    Ok(()) => JsFuture::from(w.close()).await.map(|_| ()).map_err(err),
+                    Err(e) => {
+                        let _ = JsFuture::from(w.abort()).await;
+                        Err(e)
+                    }
+                }
+            }
+            Backend::Idb(db) => {
+                let (tx, store) = Self::idb_store(db, true)?;
+                let value = match offset {
+                    // read-modify-write inside one transaction
+                    Some(off) => {
+                        let old = idb_req(&store.get(&path.into()).map_err(err)?).await?;
+                        let mut v = if old.is_undefined() || old.is_null() { Vec::new() } else { Uint8Array::new(&old).to_vec() };
+                        v.truncate(off);
+                        v.extend_from_slice(data);
+                        Uint8Array::from(&v[..])
+                    }
+                    None => Uint8Array::from(data),
+                };
+                store.put_with_key(&value, &path.into()).map_err(err)?;
+                idb_done(&tx).await
+            }
+        }
+    }
+
+    #[allow(dead_code)] // thumbnail cache pruning (render workers)
+    pub async fn remove(&self, path: &str) -> Result<(), String> {
+        match self {
+            Backend::Opfs(root) => {
+                let Some((dir, name)) = Self::parent(root, path, false).await? else { return Ok(()) };
+                match JsFuture::from(dir.remove_entry(&name)).await {
+                    Ok(_) => Ok(()),
+                    Err(e) if Reflect::get(&e, &"name".into()).ok().and_then(|n| n.as_string()).as_deref() == Some("NotFoundError") => Ok(()),
+                    Err(e) => Err(err(e)),
+                }
+            }
+            Backend::Idb(db) => {
+                let (tx, store) = Self::idb_store(db, true)?;
+                store.delete(&path.into()).map_err(err)?;
+                idb_done(&tx).await
+            }
+        }
+    }
+
+    /// Names of the files directly inside `dir`.
+    #[allow(dead_code)] // thumbnail cache pruning (render workers)
+    pub async fn list(&self, dir: &str) -> Result<Vec<String>, String> {
+        match self {
+            Backend::Opfs(root) => {
+                // `parent` of a child path is `dir` itself
+                let Some((handle, _)) = Self::parent(root, &format!("{dir}/-"), false).await? else { return Ok(Vec::new()) };
+                dir_keys(&handle).await
+            }
+            Backend::Idb(db) => {
+                let (_tx, store) = Self::idb_store(db, false)?;
+                let prefix = format!("{dir}/");
+                let range = web_sys::IdbKeyRange::bound(&prefix.as_str().into(), &format!("{prefix}\u{ffff}").into()).map_err(err)?;
+                let keys = idb_req(&store.get_all_keys_with_key(&range).map_err(err)?).await?;
+                let arr: js_sys::Array = keys.unchecked_into();
+                Ok(arr
+                    .iter()
+                    .filter_map(|k| k.as_string())
+                    .filter_map(|k| k.strip_prefix(&prefix).filter(|r| !r.contains('/')).map(str::to_string))
+                    .collect())
+            }
+        }
+    }
+
+    /// Delete everything (the "reset" escape hatch: `?reset` in the URL).
+    pub async fn clear(&self) -> Result<(), String> {
+        match self {
+            Backend::Opfs(root) => {
+                for name in dir_keys(root).await? {
+                    let opts = web_sys::FileSystemRemoveOptions::new();
+                    opts.set_recursive(true);
+                    JsFuture::from(root.remove_entry_with_options(&name, &opts)).await.map_err(err)?;
+                }
+                Ok(())
+            }
+            Backend::Idb(db) => {
+                let (tx, store) = Self::idb_store(db, true)?;
+                store.clear().map_err(err)?;
+                idb_done(&tx).await
+            }
+        }
+    }
+}
+
+/// Names of the entries in an OPFS directory.
+async fn dir_keys(dir: &FileSystemDirectoryHandle) -> Result<Vec<String>, String> {
+    let keys = dir.keys();
+    let mut out = Vec::new();
+    loop {
+        let next = JsFuture::from(keys.next().map_err(err)?).await.map_err(err)?;
+        if Reflect::get(&next, &"done".into()).ok().is_some_and(|d| d.is_truthy()) {
+            break;
+        }
+        if let Some(k) = Reflect::get(&next, &"value".into()).ok().and_then(|v| v.as_string()) {
+            out.push(k);
+        }
+    }
+    Ok(out)
+}
+
+/// Ask the browser not to evict our storage under pressure (best effort; fire and forget).
+pub fn request_persistence() {
+    if let Some(sm) = storage_manager()
+        && let Ok(p) = sm.persist()
+    {
+        wasm_bindgen_futures::spawn_local(async move {
+            let granted = JsFuture::from(p).await.ok().is_some_and(|v| v.is_truthy());
+            log::info!("lightcraft: persistent storage {}", if granted { "granted" } else { "not granted (best effort)" });
+        });
+    }
+}
diff --git a/apps/lightcraft-web/src/bench.rs b/apps/lightcraft-web/src/bench.rs
index 7ede4a7ad..90b85af84 100644
--- a/apps/lightcraft-web/src/bench.rs
+++ b/apps/lightcraft-web/src/bench.rs
@@ -65,7 +65,7 @@ impl Bench {
         if self.first_frame_ms.is_none() && self.frames > 1 {
             self.first_frame_ms = Some(now - self.start_ms);
         }
-        let idle = app.renderer.queued() == 0;
+        let idle = app.renderer.queued() == 0 && app.renderer.in_flight() == 0;
         match self.phase {
             Phase::Idle => {
                 let thumbs = app.renderer.textures.keys().filter(|s| matches!(s, Slot::Thumb(_))).count();
diff --git a/apps/lightcraft-web/src/files.rs b/apps/lightcraft-web/src/files.rs
new file mode 100644
index 000000000..18acc93d3
--- /dev/null
+++ b/apps/lightcraft-web/src/files.rs
@@ -0,0 +1,255 @@
+//! The library's small files (catalog journal, presets, view state, prefs) on the web.
+//!
+//! Browser storage is asynchronous on the main thread (OPFS writable streams, IndexedDB), but the
+//! catalog [`Store`] is synchronous. So the files are mirrored in memory: they are read once at
+//! start-up ([`Files::preload`]), every write changes the mirror immediately and marks the file
+//! dirty, and the host flushes dirty files to storage in the background ([`Files::take_dirty`],
+//! about once per frame while anything is dirty).
+//!
+//! Durability: a write is in browser storage a few milliseconds after the command that made it
+//! (one flush), not before the command returns as on the desktop. Each flushed file is replaced
+//! atomically (OPFS swap file / one IndexedDB transaction), and files are flushed in the order they
+//! were last modified, so the journal's recovery rules still hold: e.g. a new `catalog.snap` always
+//! lands before the `catalog.log` reset that follows it.
+
+use std::collections::BTreeMap;
+use std::io;
+use std::sync::{Arc, Mutex};
+
+use lightcraft_engine::catalog::Store;
+
+/// What has to be written for a dirty file.
+#[derive(Clone, Debug, PartialEq)]
+pub enum FlushOp {
+    /// Replace the whole file.
+    Write { name: String, data: Vec<u8> },
+    /// Bytes appended since the last flush, starting at `offset` (the stored file's length).
+    Append { name: String, offset: usize, data: Vec<u8> },
+}
+
+impl FlushOp {
+    pub fn name(&self) -> &str {
+        match self {
+            FlushOp::Write { name, .. } | FlushOp::Append { name, .. } => name,
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+enum Dirty {
+    Rewrite,
+    /// Only appended since the last flush; the stored file has `from` bytes.
+    Append {
+        from: usize,
+    },
+}
+
+#[derive(Default)]
+struct Mirror {
+    files: BTreeMap<String, Vec<u8>>,
+    /// Dirty files, least recently modified first.
+    dirty: Vec<(String, Dirty)>,
+}
+
+impl Mirror {
+    fn mark(&mut self, name: &str, d: Dirty) {
+        let prev = self.dirty.iter().position(|(n, _)| n == name).map(|i| self.dirty.remove(i).1);
+        let d = match (prev, d) {
+            // appends after appends keep the original offset; anything else needs a rewrite
+            (Some(Dirty::Append { from }), Dirty::Append { .. }) => Dirty::Append { from },
+            (Some(Dirty::Rewrite), _) => Dirty::Rewrite,
+            (_, d) => d,
+        };
+        self.dirty.push((name.to_string(), d));
+    }
+}
+
+/// Shared in-memory mirror of the library files (cheap to clone).
+#[derive(Clone, Default)]
+pub struct Files(Arc<Mutex<Mirror>>);
+
+impl Files {
+    fn lock(&self) -> std::sync::MutexGuard<'_, Mirror> {
+        self.0.lock().unwrap_or_else(|e| e.into_inner())
+    }
+
+    /// Set a file's stored content (start-up; not dirty).
+    pub fn preload(&self, name: &str, data: Vec<u8>) {
+        self.lock().files.insert(name.to_string(), data);
+    }
+
+    pub fn get(&self, name: &str) -> Option<Vec<u8>> {
+        self.lock().files.get(name).cloned()
+    }
+
+    /// Replace a file.
+    pub fn write(&self, name: &str, data: &[u8]) {
+        let mut m = self.lock();
+        m.files.insert(name.to_string(), data.to_vec());
+        m.mark(name, Dirty::Rewrite);
+    }
+
+    pub fn append(&self, name: &str, data: &[u8]) {
+        let mut m = self.lock();
+        let f = m.files.entry(name.to_string()).or_default();
+        let from = f.len();
+        f.extend_from_slice(data);
+        m.mark(name, Dirty::Append { from });
+    }
+
+    pub fn truncate(&self, name: &str, len: usize) {
+        let mut m = self.lock();
+        if let Some(f) = m.files.get_mut(name) {
+            f.truncate(len);
+            m.mark(name, Dirty::Rewrite);
+        }
+    }
+
+    pub fn is_dirty(&self) -> bool {
+        !self.lock().dirty.is_empty()
+    }
+
+    /// Everything to write, in modification order; the files count as clean afterwards (call
+    /// [`Files::failed`] for ops that couldn't be written).
+    pub fn take_dirty(&self) -> Vec<FlushOp> {
+        let mut m = self.lock();
+        let dirty = std::mem::take(&mut m.dirty);
+        dirty
+            .into_iter()
+            .map(|(name, d)| {
+                let data = m.files.get(&name).cloned().unwrap_or_default();
+                match d {
+                    Dirty::Append { from } if from <= data.len() => FlushOp::Append { offset: from, data: data[from..].to_vec(), name },
+                    _ => FlushOp::Write { name, data },
+                }
+            })
+            .collect()
+    }
+
+    /// A flush failed: write these files in full next time (keeps any newer modification order).
+    pub fn failed(&self, ops: &[FlushOp]) {
+        let mut m = self.lock();
+        for op in ops.iter().rev() {
+            if !m.dirty.iter().any(|(n, _)| n == op.name()) {
+                m.dirty.insert(0, (op.name().to_string(), Dirty::Rewrite));
+            } else if let Some(d) = m.dirty.iter_mut().find(|(n, _)| n == op.name()) {
+                d.1 = Dirty::Rewrite;
+            }
+        }
+    }
+
+    /// A catalog [`Store`] over these files.
+    pub fn store(&self) -> FilesStore {
+        FilesStore(self.clone())
+    }
+}
+
+/// [`Store`] implementation over [`Files`].
+pub struct FilesStore(Files);
+
+impl Store for FilesStore {
+    fn read(&mut self, name: &str) -> io::Result<Option<Vec<u8>>> {
+        Ok(self.0.get(name))
+    }
+    fn write_atomic(&mut self, name: &str, data: &[u8]) -> io::Result<()> {
+        self.0.write(name, data);
+        Ok(())
+    }
+    fn append(&mut self, name: &str, data: &[u8]) -> io::Result<()> {
+        self.0.append(name, data);
+        Ok(())
+    }
+    fn truncate(&mut self, name: &str, len: u64) -> io::Result<()> {
+        self.0.truncate(name, len as usize);
+        Ok(())
+    }
+    fn describe(&self) -> String {
+        "browser storage".into()
+    }
+}
+
+/// Names of the library files read at start-up.
+pub const LIBRARY_FILES: [&str; 6] = ["catalog.snap", "catalog.log", "presets.json", "view.json", "prefs.json", "ui.json"];
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use lightcraft_engine::Session;
+    use lightcraft_engine::library::LibraryStores;
+    use serde_json::json;
+
+    /// Apply flush ops to a "disk" map, as the backend would.
+    fn flush(files: &Files, disk: &mut BTreeMap<String, Vec<u8>>) {
+        for op in files.take_dirty() {
+            match op {
+                FlushOp::Write { name, data } => {
+                    disk.insert(name, data);
+                }
+                FlushOp::Append { name, offset, data } => {
+                    let f = disk.entry(name).or_default();
+                    assert_eq!(f.len(), offset, "append offset matches the stored length");
+                    f.extend_from_slice(&data);
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn appends_coalesce_and_rewrites_win() {
+        let f = Files::default();
+        f.preload("log", b"ab".to_vec());
+        f.append("log", b"c");
+        f.append("log", b"d");
+        assert_eq!(f.take_dirty(), vec![FlushOp::Append { name: "log".into(), offset: 2, data: b"cd".to_vec() }]);
+        assert!(!f.is_dirty());
+        f.write("snap", b"S");
+        f.truncate("log", 1);
+        f.append("log", b"x");
+        let ops = f.take_dirty();
+        assert_eq!(
+            ops,
+            vec![FlushOp::Write { name: "snap".into(), data: b"S".to_vec() }, FlushOp::Write { name: "log".into(), data: b"ax".to_vec() }]
+        );
+        f.failed(&ops);
+        assert_eq!(f.take_dirty().len(), 2);
+    }
+
+    #[test]
+    fn modification_order_puts_snapshot_before_log_reset() {
+        let f = Files::default();
+        f.append("catalog.log", b"1\n");
+        f.write("catalog.snap", b"snap");
+        f.write("catalog.log", b"");
+        let names: Vec<_> = f.take_dirty().iter().map(|o| o.name().to_string()).collect();
+        assert_eq!(names, ["catalog.snap", "catalog.log"]);
+    }
+
+    #[test]
+    fn library_survives_reload_through_flushes() {
+        let mut disk = BTreeMap::new();
+        let open = |disk: &BTreeMap<String, Vec<u8>>| {
+            let files = Files::default();
+            for (k, v) in disk {
+                files.preload(k, v.clone());
+            }
+            let mut s = Session::new();
+            let stores = LibraryStores { dir: "browser".into(), catalog: Box::new(files.store()), files: Box::new(files.store()), on_disk: false };
+            s.open_library_in(stores, true).unwrap();
+            (s, files)
+        };
+        let (mut s, files) = open(&disk);
+        assert!(!s.catalog.is_empty(), "seeded with the demo photos");
+        flush(&files, &mut disk);
+        let id = s.selection.active.unwrap();
+        s.execute("develop.set", &json!({"control": "light.exposure", "value": 1.25})).unwrap();
+        s.execute("photo.rate", &json!({"rating": 4})).unwrap();
+        s.save_view();
+        flush(&files, &mut disk);
+        let (s2, _) = open(&disk);
+        let p = s2.catalog.photo(id).unwrap();
+        assert_eq!(p.rating, 4);
+        assert_eq!(p.develop.light.exposure, 1.25);
+        assert_eq!(s2.catalog.len(), s.catalog.len(), "not seeded twice");
+        assert_eq!(s2.selection.active, s.selection.active);
+    }
+}
diff --git a/apps/lightcraft-web/src/lib.rs b/apps/lightcraft-web/src/lib.rs
index 0ab06bc94..a2c5b5898 100644
--- a/apps/lightcraft-web/src/lib.rs
+++ b/apps/lightcraft-web/src/lib.rs
@@ -1,20 +1,24 @@
 //! LightCraft in the browser: the same egui UI as the desktop app, compiled to WASM.
 //!
-//! Build with `cargo xtask web` (→ `target/web/`), then serve that folder over HTTP.
-//! Differences from the desktop host:
-//! - no filesystem: photos picked (File ▸ Add Photos…) or dropped onto the page are held in
-//!   memory ([`store::MemStore`]) and decoded from bytes;
-//! - export downloads the file through the browser;
+//! Build with `cargo xtask web` (→ `target/web/`), then serve that folder over HTTP (see
+//! `docs/web.md`). Differences from the desktop host:
+//! - the library lives in browser storage (OPFS, or IndexedDB where OPFS can't be written from the
+//!   main thread): catalog journal, presets, view state and prefs are mirrored in memory and
+//!   flushed in the background ([`files`]); imported originals are stored by content hash
+//!   ([`store`]); a new library starts with the procedural demo photos;
 //! - renders run inline on the main thread (one job per frame) — no worker threads yet;
-//! - no persistence yet (UI prefs and the catalog reset on reload);
-//! - the procedural demo library is loaded at start so the page is never empty.
+//! - export downloads the file through the browser.
 //!
 //! `?bench` in the URL runs a scripted first-paint / slider-latency measurement and logs it to the
-//! console (see [`bench`]).
+//! console (see [`bench`]); `?store=idb` forces the IndexedDB backend, `?store=memory` disables
+//! persistence and `?reset` clears the stored library.
 #![forbid(unsafe_code)]
 
 pub mod bench;
+pub mod files;
 pub mod store;
 
+#[cfg(target_arch = "wasm32")]
+mod backend;
 #[cfg(target_arch = "wasm32")]
 mod web;
diff --git a/apps/lightcraft-web/src/store.rs b/apps/lightcraft-web/src/store.rs
index acac328e2..4b395c57b 100644
--- a/apps/lightcraft-web/src/store.rs
+++ b/apps/lightcraft-web/src/store.rs
@@ -1,6 +1,13 @@
-//! In-memory "filesystem" for the web build: files picked or dropped in the browser are kept as
-//! bytes under synthetic paths (`mem/<n>/<name>`), and the engine reads them through the same
-//! [`FileLoader`]/[`FileProbe`] hooks the desktop app backs with `std::fs`.
+//! Imported originals on the web. There is no filesystem: the bytes of every photo picked or
+//! dropped in the browser are stored in browser storage as `originals/<content hash>`, and the
+//! catalog refers to them by a synthetic path `web/<content hash>/<file name>` (so the decoders
+//! still see the file's extension).
+//!
+//! The engine reads files synchronously ([`FileLoader`]/[`FileProbe`]), so the main thread keeps
+//! the bytes it needs in a bounded memory cache ([`Originals`]). A miss (e.g. after a reload)
+//! queues an asynchronous load from storage ([`Originals::take_wanted`]) and fails for now; the
+//! host prefetches the active photo's original so commands that need its pixels (auto tone,
+//! export) find it. Render workers read originals from storage themselves.
 
 use std::collections::HashMap;
 use std::sync::{Arc, Mutex};
@@ -8,37 +15,119 @@ use std::sync::{Arc, Mutex};
 use lightcraft_engine::files::{load_bytes, probe_bytes};
 use lightcraft_engine::media::{FileLoader, FileProbe};
 
-#[derive(Default)]
-struct Inner {
-    files: HashMap<String, Arc<[u8]>>,
-    /// Paths added since the last [`MemStore::take_pending`] (to import on the next frame).
-    pending: Vec<String>,
-    next: u64,
+/// Prefix of the catalog paths of files kept in browser storage.
+pub const PATH_PREFIX: &str = "web/";
+
+/// Memory budget for original bytes on the main thread.
+const MEM_BUDGET: usize = 768 << 20;
+
+/// Catalog path for a stored original.
+pub fn original_path(hash: &str, name: &str) -> String {
+    let name = name.rsplit(['/', '\\']).next().filter(|n| !n.is_empty()).unwrap_or("photo");
+    format!("{PATH_PREFIX}{hash}/{name}")
 }
 
-/// Shared, cloneable byte store. File reads in the browser are async, so additions arrive from
-/// promise callbacks and are picked up by the app on its next frame.
-#[derive(Clone, Default)]
-pub struct MemStore(Arc<Mutex<Inner>>);
+/// The content hash in a path made by [`original_path`].
+pub fn hash_of_path(path: &str) -> Option<&str> {
+    let (hash, _) = path.strip_prefix(PATH_PREFIX)?.split_once('/')?;
+    (!hash.is_empty()).then_some(hash)
+}
 
-impl MemStore {
+/// Storage key of an original.
+pub fn storage_key(hash: &str) -> String {
+    format!("originals/{hash}")
+}
+
+/// Hash of a file's bytes (the same as the catalog's `content_hash`).
+pub fn content_hash(bytes: &[u8]) -> String {
+    lightcraft_preview::hash_bytes(bytes).to_string()
+}
+
+#[derive(Default)]
+struct Inner {
+    /// hash → (bytes, last use).
+    bytes: HashMap<String, (Arc<[u8]>, u64)>,
+    total: usize,
+    clock: u64,
+    /// Catalog paths stored since the last [`Originals::take_pending`] (to import on the next frame).
+    pending: Vec<String>,
+    /// Hashes to load from storage.
+    wanted: Vec<String>,
+    /// Hashes being loaded.
+    loading: std::collections::HashSet<String>,
+}
+
+/// Original bytes in memory, shared between the engine hooks and the host (cheap to clone).
+#[derive(Clone, Default)]
+pub struct Originals(Arc<Mutex<Inner>>);
+
+impl Originals {
     fn lock(&self) -> std::sync::MutexGuard<'_, Inner> {
         self.0.lock().unwrap_or_else(|e| e.into_inner())
     }
 
-    /// Store a file's bytes; returns its synthetic path and queues it for import.
-    pub fn add(&self, name: &str, bytes: Vec<u8>) -> String {
+    /// Keep `bytes` (hash `hash`) in memory, evicting the least recently used others if needed.
+    pub fn insert(&self, hash: &str, bytes: Arc<[u8]>) {
         let mut g = self.lock();
-        g.next += 1;
-        let name = name.rsplit(['/', '\\']).next().filter(|n| !n.is_empty()).unwrap_or("photo");
-        let path = format!("mem/{}/{name}", g.next);
-        g.files.insert(path.clone(), bytes.into());
-        g.pending.push(path.clone());
+        g.clock += 1;
+        let clock = g.clock;
+        g.loading.remove(hash);
+        if let Some(old) = g.bytes.insert(hash.to_string(), (bytes.clone(), clock)) {
+            g.total -= old.0.len();
+        }
+        g.total += bytes.len();
+        while g.total > MEM_BUDGET && g.bytes.len() > 1 {
+            let Some(oldest) = g.bytes.iter().filter(|(h, _)| *h != hash).min_by_key(|(_, v)| v.1).map(|(h, _)| h.clone()) else { break };
+            if let Some(v) = g.bytes.remove(&oldest) {
+                g.total -= v.0.len();
+            }
+        }
+    }
+
+    /// A file was stored (bytes in storage and in memory): queue its path for import.
+    pub fn added(&self, name: &str, hash: &str, bytes: Arc<[u8]>) -> String {
+        self.insert(hash, bytes);
+        let path = original_path(hash, name);
+        self.lock().pending.push(path.clone());
         path
     }
 
+    /// Bytes for a catalog path, if in memory. A miss for a stored original queues its load.
     pub fn get(&self, path: &str) -> Option<Arc<[u8]>> {
-        self.lock().files.get(path).cloned()
+        let hash = hash_of_path(path)?;
+        let mut g = self.lock();
+        g.clock += 1;
+        let clock = g.clock;
+        if let Some(v) = g.bytes.get_mut(hash) {
+            v.1 = clock;
+            return Some(v.0.clone());
+        }
+        if !g.loading.contains(hash) && !g.wanted.iter().any(|w| w == hash) {
+            g.wanted.push(hash.to_string());
+        }
+        None
+    }
+
+    pub fn contains(&self, path: &str) -> bool {
+        hash_of_path(path).is_some_and(|h| self.lock().bytes.contains_key(h))
+    }
+
+    /// Ask for the original behind `path` to be loaded into memory (no-op if it's there).
+    pub fn prefetch(&self, path: &str) {
+        let _ = self.get(path);
+    }
+
+    /// Hashes to load from storage (they count as loading until [`Originals::insert`] or
+    /// [`Originals::load_failed`]).
+    pub fn take_wanted(&self) -> Vec<String> {
+        let mut g = self.lock();
+        let w = std::mem::take(&mut g.wanted);
+        g.loading.extend(w.iter().cloned());
+        w
+    }
+
+    pub fn load_failed(&self, hash: &str) {
+        self.lock().loading.remove(hash);
     }
 
     /// Paths added since the last call.
@@ -46,24 +135,22 @@ impl MemStore {
         std::mem::take(&mut self.lock().pending)
     }
 
-    pub fn len(&self) -> usize {
-        self.lock().files.len()
+    /// (originals in memory, bytes).
+    pub fn usage(&self) -> (usize, usize) {
+        let g = self.lock();
+        (g.bytes.len(), g.total)
     }
 
-    pub fn is_empty(&self) -> bool {
-        self.len() == 0
-    }
-
-    /// Engine hooks that decode/probe from this store.
+    /// Engine hooks that decode/probe from memory.
     pub fn hooks(&self) -> (FileLoader, FileProbe) {
         let s = self.clone();
         let loader: FileLoader = Arc::new(move |path: &str, max_edge: usize| {
-            let bytes = s.get(path).ok_or_else(|| format!("{path}: not in memory"))?;
+            let bytes = s.get(path).ok_or_else(|| format!("{path}: the original is still loading from browser storage"))?;
             load_bytes(&bytes, max_edge)
         });
         let s = self.clone();
         let probe: FileProbe = Arc::new(move |path: &str| {
-            let bytes = s.get(path).ok_or_else(|| format!("{path}: not in memory"))?;
+            let bytes = s.get(path).ok_or_else(|| format!("{path}: not loaded"))?;
             probe_bytes(path, &bytes)
         });
         (loader, probe)
@@ -94,41 +181,54 @@ pub fn mime_for(name: &str) -> &'static str {
     }
 }
 
+#[cfg(test)]
+pub(crate) fn png_bytes(w: usize, h: usize) -> Vec<u8> {
+    let bytes: Vec<u8> = (0..w * h).flat_map(|i| [(i * 7) as u8, 128, 200, 255]).collect();
+    let img = lightcraft_raster::Rgba8::from_bytes(w, h, &bytes).unwrap();
+    lightcraft_codecs::encode_png(&lightcraft_codecs::EncodeImage::rgba8(&img), &lightcraft_codecs::EncodeMeta::default()).unwrap()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
     use serde_json::json;
 
-    fn png_bytes(w: usize, h: usize) -> Vec<u8> {
-        let bytes: Vec<u8> = (0..w * h).flat_map(|i| [(i * 7) as u8, 128, 200, 255]).collect();
-        let img = lightcraft_raster::Rgba8::from_bytes(w, h, &bytes).unwrap();
-        lightcraft_codecs::encode_png(&lightcraft_codecs::EncodeImage::rgba8(&img), &lightcraft_codecs::EncodeMeta::default()).unwrap()
+    #[test]
+    fn paths() {
+        let p = original_path("abc123", "C:\\x\\a.png");
+        assert_eq!(p, "web/abc123/a.png");
+        assert_eq!(hash_of_path(&p), Some("abc123"));
+        assert_eq!(hash_of_path("mem/1/a.png"), None);
+        assert_eq!(storage_key("abc"), "originals/abc");
     }
 
     #[test]
-    fn add_take_get() {
-        let s = MemStore::default();
-        let a = s.add("C:\\x\\a.png", vec![1, 2]);
-        let b = s.add("a.png", vec![3]);
-        assert_ne!(a, b, "same name twice gets distinct paths");
-        assert!(a.ends_with("/a.png"));
-        assert_eq!(s.take_pending(), vec![a.clone(), b]);
-        assert!(s.take_pending().is_empty());
-        assert_eq!(&*s.get(&a).unwrap(), &[1, 2]);
-        assert_eq!(s.len(), 2);
+    fn miss_queues_a_load_once() {
+        let s = Originals::default();
+        assert!(s.get("web/h1/a.png").is_none());
+        assert!(s.get("web/h1/a.png").is_none());
+        assert_eq!(s.take_wanted(), vec!["h1".to_string()]);
+        assert!(s.get("web/h1/a.png").is_none(), "still loading");
+        assert!(s.take_wanted().is_empty());
+        s.insert("h1", Arc::from(vec![1u8, 2]));
+        assert_eq!(&*s.get("web/h1/b.jpg").unwrap(), &[1, 2]);
     }
 
     #[test]
     fn import_and_render_from_memory() {
-        let store = MemStore::default();
+        let store = Originals::default();
         let mut session = lightcraft_engine::Session::new();
         store.install(&mut session);
-        let path = store.add("tiny.png", png_bytes(40, 30));
+        let bytes = png_bytes(40, 30);
+        let hash = content_hash(&bytes);
+        let path = store.added("tiny.png", &hash, bytes.into());
+        assert_eq!(store.take_pending(), vec![path.clone()]);
         let r = session.execute("library.import", &json!({"paths": [path]})).unwrap();
         let id = lightcraft_engine::catalog::PhotoId(r["imported"][0].as_u64().unwrap());
         let p = session.catalog.photo(id).unwrap();
         assert_eq!((p.width, p.height), (40, 30));
         assert_eq!(p.file_name, "tiny.png");
+        assert_eq!(p.content_hash.as_deref(), Some(hash.as_str()), "storage key = catalog content hash");
         let out = session.render_now(id, 20, 20).unwrap();
         assert_eq!(out.image.width, 20);
     }
diff --git a/apps/lightcraft-web/src/web.rs b/apps/lightcraft-web/src/web.rs
index c902a2589..d11343b0b 100644
--- a/apps/lightcraft-web/src/web.rs
+++ b/apps/lightcraft-web/src/web.rs
@@ -1,17 +1,32 @@
-//! Browser host: starts eframe on `<canvas id="lightcraft_canvas">`, wires the in-memory store to
-//! the file picker and drag-and-drop, and turns exports into downloads.
+//! Browser host: opens the library in browser storage, starts eframe on
+//! `<canvas id="lightcraft_canvas">`, wires the file picker and drag-and-drop, and turns exports
+//! into downloads.
+
+use std::cell::Cell;
+use std::rc::Rc;
+use std::sync::Arc;
 
 use lightcraft_engine::Session;
-use lightcraft_ui_egui::{LightcraftApp, Services};
+use lightcraft_engine::catalog::Source;
+use lightcraft_engine::library::LibraryStores;
+use lightcraft_ui_egui::{LightcraftApp, Services, UiState};
 use serde_json::json;
 use wasm_bindgen::JsCast;
 use wasm_bindgen::prelude::*;
 
+use crate::backend::Backend;
 use crate::bench::Bench;
-use crate::store::{MemStore, download_name};
+use crate::files::{Files, FlushOp, LIBRARY_FILES};
+use crate::store::{Originals, content_hash, download_name, storage_key};
 
 const ACCEPT: &str = ".jpg,.jpeg,.png,.tif,.tiff,.webp,.dng,.cr2,.nef,.arw,.psd,.jxl,.gif,.bmp";
 
+/// Where the library files live in browser storage.
+const LIBRARY_DIR: &str = "library";
+
+/// How often view state and UI prefs are saved (ms).
+const SAVE_EVERY_MS: f64 = 1000.0;
+
 fn window() -> web_sys::Window {
     web_sys::window().expect("no window")
 }
@@ -27,19 +42,62 @@ fn set_status(text: &str) {
     }
 }
 
-/// Read a browser `File` into the store.
-async fn read_file(store: MemStore, file: web_sys::File, ctx: egui::Context) {
-    match wasm_bindgen_futures::JsFuture::from(file.array_buffer()).await {
-        Ok(buf) => {
-            store.add(&file.name(), js_sys::Uint8Array::new(&buf).to_vec());
-            ctx.request_repaint();
+fn library_key(name: &str) -> String {
+    format!("{LIBRARY_DIR}/{name}")
+}
+
+/// URL options (`?bench&store=idb&reset`).
+struct Options {
+    bench: bool,
+    /// "opfs" (default: OPFS, falling back to IndexedDB), "idb" or "memory".
+    store: String,
+    reset: bool,
+}
+
+impl Options {
+    fn from_url() -> Options {
+        let q = window().location().search().unwrap_or_default();
+        let params: Vec<(String, String)> = q
+            .trim_start_matches('?')
+            .split('&')
+            .filter(|p| !p.is_empty())
+            .map(|p| match p.split_once('=') {
+                Some((k, v)) => (k.to_string(), v.to_string()),
+                None => (p.to_string(), String::new()),
+            })
+            .collect();
+        let get = |k: &str| params.iter().find(|(n, _)| n == k).map(|(_, v)| v.clone());
+        Options {
+            bench: get("bench").is_some(),
+            store: get("store").filter(|s| s == "idb" || s == "memory").unwrap_or_else(|| "opfs".into()),
+            reset: get("reset").is_some(),
         }
+    }
+}
+
+/// Store a picked/dropped file (bytes into browser storage, then queue its import).
+async fn store_file(originals: Originals, backend: Option<Backend>, name: String, bytes: Vec<u8>, ctx: egui::Context) {
+    let hash = content_hash(&bytes);
+    if let Some(b) = &backend
+        && let Err(e) = b.write(&storage_key(&hash), &bytes).await
+    {
+        // still importable for this session; it won't survive a reload
+        log::error!("storing {name} in browser storage failed: {e}");
+    }
+    originals.added(&name, &hash, Arc::from(bytes));
+    ctx.request_repaint();
+}
+
+/// Read a browser `File` into storage.
+async fn read_file(originals: Originals, backend: Option<Backend>, file: web_sys::File, ctx: egui::Context) {
+    match wasm_bindgen_futures::JsFuture::from(file.array_buffer()).await {
+        Ok(buf) => store_file(originals, backend, file.name(), js_sys::Uint8Array::new(&buf).to_vec(), ctx).await,
         Err(e) => log::warn!("reading {}: {e:?}", file.name()),
     }
 }
 
-/// Show the browser's open dialog; picked files land in the store asynchronously.
-fn open_picker(store: MemStore, ctx: egui::Context) {
+/// Show the browser's open dialog; picked files are imported asynchronously.
+fn open_picker(originals: Originals, backend: Option<Backend>, ctx: egui::Context) {
     let Some(doc) = window().document() else { return };
     let Ok(input) = doc.create_element("input").map(|e| e.unchecked_into::<web_sys::HtmlInputElement>()) else { return };
     input.set_type("file");
@@ -50,7 +108,7 @@ fn open_picker(store: MemStore, ctx: egui::Context) {
         if let Some(files) = inp.files() {
             for i in 0..files.length() {
                 if let Some(f) = files.get(i) {
-                    wasm_bindgen_futures::spawn_local(read_file(store.clone(), f, ctx.clone()));
+                    wasm_bindgen_futures::spawn_local(read_file(originals.clone(), backend.clone(), f, ctx.clone()));
                 }
             }
         }
@@ -83,10 +141,10 @@ fn download(path: &str, bytes: &[u8]) -> Result<(), String> {
     Ok(())
 }
 
-fn services(store: MemStore, ctx: egui::Context) -> Services {
+fn services(originals: Originals, backend: Option<Backend>, ctx: egui::Context) -> Services {
     Services {
         pick_files: Some(Box::new(move || {
-            open_picker(store.clone(), ctx.clone());
+            open_picker(originals.clone(), backend.clone(), ctx.clone());
             Vec::new() // files arrive asynchronously and are imported on a later frame
         })),
         // Preset files: browser pickers are asynchronous; not wired on the web yet.
@@ -99,44 +157,145 @@ fn services(store: MemStore, ctx: egui::Context) -> Services {
     }
 }
 
+/// Write dirty library files to storage, in order, until none are left.
+async fn flush(files: Files, backend: Backend, flushing: Rc<Cell<bool>>) {
+    'outer: loop {
+        let ops = files.take_dirty();
+        if ops.is_empty() {
+            break;
+        }
+        for (i, op) in ops.iter().enumerate() {
+            let r = match op {
+                FlushOp::Write { name, data } => backend.write(&library_key(name), data).await,
+                FlushOp::Append { name, offset, data } => backend.write_at(&library_key(name), Some(*offset), data).await,
+            };
+            if let Err(e) = r {
+                log::error!("saving {} failed: {e}", op.name());
+                files.failed(&ops[i..]);
+                break 'outer;
+            }
+        }
+    }
+    flushing.set(false);
+}
+
+/// Everything loaded before the UI starts.
+struct Boot {
+    opts: Options,
+    backend: Option<Backend>,
+    files: Files,
+}
+
+async fn boot(opts: Options) -> Boot {
+    let t0 = perf_now();
+    let backend = if opts.store == "memory" {
+        None
+    } else {
+        match Backend::open(opts.store == "idb").await {
+            Ok(b) => Some(b),
+            Err(e) => {
+                log::error!("no browser storage ({e}); this session won't be saved");
+                None
+            }
+        }
+    };
+    let files = Files::default();
+    if let Some(b) = &backend {
+        if opts.reset {
+            match b.clear().await {
+                Ok(()) => log::info!("lightcraft: storage cleared (?reset)"),
+                Err(e) => log::error!("clearing storage: {e}"),
+            }
+        }
+        for name in LIBRARY_FILES {
+            match b.read(&library_key(name)).await {
+                Ok(Some(data)) => files.preload(name, data),
+                Ok(None) => {}
+                Err(e) => log::error!("reading {name}: {e}"),
+            }
+        }
+        crate::backend::request_persistence();
+    }
+    log::info!("lightcraft: storage {} opened in {:.0} ms", backend.as_ref().map_or("memory", |b| b.kind()), perf_now() - t0);
+    Boot { opts, backend, files }
+}
+
 struct WebApp {
     app: LightcraftApp,
-    store: MemStore,
+    originals: Originals,
+    files: Files,
+    backend: Option<Backend>,
+    flushing: Rc<Cell<bool>>,
     bench: Option<Bench>,
     first_frame_logged: bool,
+    last_save: f64,
+    ui_written: Vec<u8>,
 }
 
 impl WebApp {
-    fn new(cc: &eframe::CreationContext<'_>, bench: bool) -> Self {
-        let store = MemStore::default();
+    fn new(cc: &eframe::CreationContext<'_>, boot: Boot) -> Self {
+        let Boot { opts, backend, files } = boot;
+        let originals = Originals::default();
         let t = perf_now();
-        let mut session = Session::with_demo();
-        log::info!("lightcraft: wasm started at {:.0} ms; demo library built in {:.0} ms", t, perf_now() - t);
-        store.install(&mut session);
+        let mut session = Session::new();
+        originals.install(&mut session);
+        let stores = LibraryStores {
+            dir: format!("browser:{}/{LIBRARY_DIR}", backend.as_ref().map_or("memory", |b| b.kind())).into(),
+            catalog: Box::new(files.store()),
+            files: Box::new(files.store()),
+            on_disk: false,
+        };
+        match session.open_library_in(stores, true).cloned() {
+            Ok(r) => log::info!(
+                "lightcraft: library {} in {:.0} ms ({} photos; snapshot seq {}, {} ops replayed)",
+                if r.created { "created" } else { "loaded" },
+                perf_now() - t,
+                session.catalog.len(),
+                r.snapshot_seq,
+                r.replayed
+            ),
+            Err(e) => {
+                log::error!("opening the library failed: {e}; starting a temporary one");
+                session = Session::with_demo();
+                originals.install(&mut session);
+            }
+        }
         // previews are large (≤ 2560 px, f32): keep few in a 32-bit address space
         session.media.preview_capacity = 3;
-        let mut app = LightcraftApp::new(session, services(store.clone(), cc.egui_ctx.clone()));
+        let mut app = LightcraftApp::new(session, services(originals.clone(), backend.clone(), cc.egui_ctx.clone()));
+        let ui_written = files.get("ui.json").unwrap_or_default();
+        if let Ok(ui) = serde_json::from_slice::<UiState>(&ui_written) {
+            app.ui = ui;
+        }
         app.ui = app.ui.sanitized();
+        CTX.with(|c| *c.borrow_mut() = Some(cc.egui_ctx.clone()));
         let origin = lightcraft_ui_egui::now_ms() - perf_now();
-        WebApp { app, store, bench: bench.then(|| Bench::new(origin)), first_frame_logged: false }
+        WebApp {
+            app,
+            originals,
+            files,
+            backend,
+            flushing: Rc::new(Cell::new(false)),
+            bench: opts.bench.then(|| Bench::new(origin)),
+            first_frame_logged: false,
+            last_save: 0.0,
+            ui_written,
+        }
     }
 
     fn import_dropped(&mut self, ctx: &egui::Context) {
         let dropped = ctx.input(|i| i.raw.dropped_files.clone());
         for f in dropped {
-            let (store, ctx) = (self.store.clone(), ctx.clone());
+            let (originals, backend, ctx) = (self.originals.clone(), self.backend.clone(), ctx.clone());
             wasm_bindgen_futures::spawn_local(async move {
                 let name = f.path().to_string_lossy().to_string();
                 match f.bytes_async().await {
-                    Ok(bytes) => {
-                        store.add(&name, bytes);
-                        ctx.request_repaint();
-                    }
+                    Ok(bytes) => store_file(originals, backend, name, bytes, ctx).await,
                     Err(e) => log::warn!("reading dropped {name}: {e}"),
                 }
             });
         }
-        let paths = self.store.take_pending();
+        let paths = self.originals.take_pending();
         if !paths.is_empty() {
             let n = paths.len();
             match self.app.run("library.import", json!({"paths": paths})) {
@@ -145,12 +304,114 @@ impl WebApp {
             }
         }
     }
+
+    /// Load originals the main thread asked for (and the active photo's, ahead of time).
+    fn load_originals(&mut self, ctx: &egui::Context) {
+        if let Some(id) = self.app.session.selection.active
+            && let Some(Source::File { path }) = self.app.session.catalog.photo(id).map(|p| &p.source)
+            && !self.originals.contains(path)
+        {
+            self.originals.prefetch(path);
+        }
+        let Some(b) = &self.backend else { return };
+        for hash in self.originals.take_wanted() {
+            let (b, originals, ctx) = (b.clone(), self.originals.clone(), ctx.clone());
+            wasm_bindgen_futures::spawn_local(async move {
+                match b.read(&storage_key(&hash)).await {
+                    Ok(Some(bytes)) => {
+                        originals.insert(&hash, Arc::from(bytes));
+                        ctx.request_repaint();
+                    }
+                    Ok(None) => {
+                        log::warn!("original {hash} is not in browser storage");
+                        originals.load_failed(&hash);
+                    }
+                    Err(e) => {
+                        log::warn!("loading original {hash}: {e}");
+                        originals.load_failed(&hash);
+                    }
+                }
+            });
+        }
+    }
+
+    /// Save view state and UI prefs now and then; flush dirty files.
+    fn save(&mut self) {
+        let now = perf_now();
+        if now - self.last_save >= SAVE_EVERY_MS {
+            self.last_save = now;
+            self.app.session.save_view();
+            if let Ok(ui) = serde_json::to_vec_pretty(&self.app.ui)
+                && ui != self.ui_written
+            {
+                self.files.write("ui.json", &ui);
+                self.ui_written = ui;
+            }
+        }
+        if let Some(b) = &self.backend
+            && self.files.is_dirty()
+            && !self.flushing.get()
+        {
+            self.flushing.set(true);
+            wasm_bindgen_futures::spawn_local(flush(self.files.clone(), b.clone(), self.flushing.clone()));
+        }
+    }
+}
+
+type Reply = (js_sys::Function, js_sys::Function);
+
+thread_local! {
+    /// Commands from JavaScript ([`command`]), run on the next frame.
+    static INBOX: std::cell::RefCell<Vec<(String, String, Reply)>> = const { std::cell::RefCell::new(Vec::new()) };
+    static CTX: std::cell::RefCell<Option<egui::Context>> = const { std::cell::RefCell::new(None) };
+}
+
+/// Run a command by id from JavaScript (automation, tests): `await command("library.info", "{}")`
+/// resolves to the result as JSON text, or rejects with the error. Besides every engine/UI
+/// command, `web.stats` reports the browser host's state (storage, originals in memory).
+#[wasm_bindgen]
+pub fn command(id: String, params: String) -> js_sys::Promise {
+    js_sys::Promise::new(&mut |resolve, reject| {
+        INBOX.with(|q| q.borrow_mut().push((id.clone(), params.clone(), (resolve, reject))));
+        CTX.with(|c| c.borrow().as_ref().map(|c| c.request_repaint()));
+    })
+}
+
+impl WebApp {
+    fn web_stats(&self) -> serde_json::Value {
+        let (orig_n, orig_bytes) = self.originals.usage();
+        json!({
+            "storage": self.backend.as_ref().map_or("memory", |b| b.kind()),
+            "dirty": self.files.is_dirty(),
+            "flushing": self.flushing.get(),
+            "originalsInMemory": orig_n,
+            "originalBytesInMemory": orig_bytes,
+            "renderQueue": self.app.renderer.queued(),
+            "rendersInFlight": self.app.renderer.in_flight(),
+            "thumbTextures": self.app.renderer.thumb_textures(),
+        })
+    }
+
+    fn run_inbox(&mut self) {
+        let inbox = INBOX.with(|q| std::mem::take(&mut *q.borrow_mut()));
+        for (id, params, (resolve, reject)) in inbox {
+            let params: serde_json::Value = serde_json::from_str(&params).unwrap_or(json!({}));
+            let r = if id == "web.stats" { Ok(self.web_stats()) } else { self.app.run(&id, params) };
+            let _ = match r {
+                Ok(v) => resolve.call1(&JsValue::NULL, &v.to_string().into()),
+                Err(e) => reject.call1(&JsValue::NULL, &e.into()),
+            };
+        }
+    }
 }
 
 impl eframe::App for WebApp {
     fn logic(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
+        self.run_inbox();
         self.import_dropped(ctx);
+        self.load_originals(ctx);
         self.app.logic(ctx);
+        self.save();
         if let Some(b) = self.bench.as_mut()
             && let Some(report) = b.step(&mut self.app)
         {
@@ -176,11 +437,12 @@ impl eframe::App for WebApp {
     }
 }
 
-#[wasm_bindgen(start)]
+/// Start the app (called by `index.html` after instantiating the module).
+#[wasm_bindgen]
 pub fn start() {
     eframe::WebLogger::init(log::LevelFilter::Info).ok();
     log::info!("lightcraft: wasm instantiated at {:.0} ms", perf_now());
-    let bench = window().location().search().is_ok_and(|q| q.contains("bench"));
+    let opts = Options::from_url();
     wasm_bindgen_futures::spawn_local(async move {
         let Some(canvas) =
             window().document().and_then(|d| d.get_element_by_id("lightcraft_canvas")).and_then(|e| e.dyn_into::<web_sys::HtmlCanvasElement>().ok())
@@ -188,8 +450,9 @@ pub fn start() {
             log::error!("missing <canvas id=\"lightcraft_canvas\">");
             return;
         };
+        let boot = boot(opts).await;
         let runner = eframe::WebRunner::new();
-        let r = runner.start(canvas, eframe::WebOptions::default(), Box::new(move |cc| Ok(Box::new(WebApp::new(cc, bench))))).await;
+        let r = runner.start(canvas, eframe::WebOptions::default(), Box::new(move |cc| Ok(Box::new(WebApp::new(cc, boot))))).await;
         match r {
             Ok(()) => {
                 if let Some(el) = window().document().and_then(|d| d.get_element_by_id("lightcraft_loading")) {
diff --git a/crates/engine/src/import.rs b/crates/engine/src/import.rs
index 3aac02034..e255b0e00 100644
--- a/crates/engine/src/import.rs
+++ b/crates/engine/src/import.rs
@@ -152,7 +152,10 @@ fn copy_into_library(lib: &Path, src: &str, date: &str) -> Result<String, String
 
 /// Import files/folders. See the module docs.
 pub fn import(s: &mut Session, paths: &[String], mode: ImportMode) -> crate::Result<ImportReport> {
-    let lib_dir = s.library.as_ref().map(|l| l.dir.clone());
+    // Only a library on disk has an `Originals/` folder. The browser build keeps the bytes of every
+    // added file in its own storage already, so "copy" there means "add".
+    let mode = if mode == ImportMode::Copy && s.library.as_ref().is_some_and(|l| !l.on_disk) { ImportMode::Add } else { mode };
+    let lib_dir = s.library.as_ref().filter(|l| l.on_disk).map(|l| l.dir.clone());
     if mode == ImportMode::Copy && lib_dir.is_none() {
         return Err(crate::EngineError::Other("copying into the library needs an open library".into()));
     }
diff --git a/crates/engine/src/library.rs b/crates/engine/src/library.rs
index e9979354c..d0fd0ac3b 100644
--- a/crates/engine/src/library.rs
+++ b/crates/engine/src/library.rs
@@ -10,6 +10,9 @@
 //!   Originals/     (photos imported with "copy into library")
 //! ```
 //!
+//! The files live in [`Store`]s: a directory ([`FsStore`]) natively, or any other implementation
+//! via [`Session::open_library_in`] (the browser build keeps them in OPFS or IndexedDB).
+//!
 //! Every top-level [`Session::execute`] persists the ops it produced (fsynced) before returning,
 //! so a crash loses at most the command in flight. The log is compacted into a snapshot when it
 //! grows (see [`lightcraft_catalog::SnapshotPolicy`]) and on [`Session::close_library`].
@@ -36,13 +39,32 @@ pub fn default_dir() -> Option<PathBuf> {
 }
 
 pub struct Library {
+    /// The library directory, or a descriptive pseudo-path when the library isn't on disk.
     pub dir: PathBuf,
+    /// The library is a real directory: photos can be copied into `Originals/` and thumbnails are
+    /// cached in `thumbs/`.
+    pub on_disk: bool,
     journal: Journal,
+    /// presets.json, view.json, prefs.json.
+    files: Box<dyn Store>,
     /// What happened when the catalog was loaded.
     pub report: LoadReport,
     /// Last persistence error (shown by the UI; ops stay pending and are retried).
     pub last_error: Option<String>,
     presets_written: String,
+    view_written: Vec<u8>,
+}
+
+/// Where a library's files live (see [`Session::open_library_in`]).
+pub struct LibraryStores {
+    /// Shown as the library's location (`library.info`).
+    pub dir: PathBuf,
+    /// The catalog journal (`catalog.snap`, `catalog.log`).
+    pub catalog: Box<dyn Store>,
+    /// presets.json, view.json, prefs.json.
+    pub files: Box<dyn Store>,
+    /// `dir` is a real directory (enables `Originals/` copies and the `thumbs/` disk cache).
+    pub on_disk: bool,
 }
 
 #[derive(Default, Serialize, Deserialize)]
@@ -87,8 +109,8 @@ fn presets_json(presets: &[Preset]) -> String {
     serde_json::to_string_pretty(&f).unwrap_or_default()
 }
 
-fn write_atomic(dir: &Path, name: &str, data: &[u8]) -> std::io::Result<()> {
-    FsStore::open(dir)?.write_atomic(name, data)
+fn read_json<T: serde::de::DeserializeOwned>(store: &mut dyn Store, name: &str) -> Option<T> {
+    store.read(name).ok().flatten().and_then(|b| serde_json::from_slice(&b).ok())
 }
 
 impl Session {
@@ -96,8 +118,16 @@ impl Session {
     /// `seed_demo`, a newly created library starts with the procedural demo photos.
     pub fn open_library(&mut self, dir: impl AsRef<Path>, seed_demo: bool) -> Result<&LoadReport> {
         let dir = dir.as_ref().to_path_buf();
-        let store = FsStore::open(&dir).map_err(|e| EngineError::Other(format!("can't open library {}: {e}", dir.display())))?;
-        let (mut journal, catalog, report) = Journal::open(Box::new(store))?;
+        let open = || FsStore::open(&dir).map_err(|e| EngineError::Other(format!("can't open library {}: {e}", dir.display())));
+        let stores = LibraryStores { catalog: Box::new(open()?), files: Box::new(open()?), on_disk: true, dir: dir.clone() };
+        self.open_library_in(stores, seed_demo)
+    }
+
+    /// Open (or create) a library whose files live in `stores` (e.g. browser storage), replacing
+    /// this session's catalog.
+    pub fn open_library_in(&mut self, stores: LibraryStores, seed_demo: bool) -> Result<&LoadReport> {
+        let LibraryStores { dir, catalog, mut files, on_disk } = stores;
+        let (mut journal, catalog, report) = Journal::open(catalog)?;
         self.catalog = catalog;
         self.undo.clear();
         self.redo.clear();
@@ -110,9 +140,7 @@ impl Session {
             journal.snapshot(&self.catalog)?;
         }
         // presets
-        if let Ok(bytes) = std::fs::read(dir.join("presets.json"))
-            && let Ok(f) = serde_json::from_slice::<PresetsFile>(&bytes)
-        {
+        if let Some(f) = read_json::<PresetsFile>(files.as_mut(), "presets.json") {
             for p in &mut self.presets {
                 p.favorite = p.builtin && f.favorites.contains(&p.id) || (!p.builtin && p.favorite);
             }
@@ -123,12 +151,9 @@ impl Session {
             }
         }
         // preferences
-        self.xmp =
-            std::fs::read(dir.join("prefs.json")).ok().and_then(|b| serde_json::from_slice::<PrefsFile>(&b).ok()).map(|p| p.xmp).unwrap_or_default();
+        self.xmp = read_json::<PrefsFile>(files.as_mut(), "prefs.json").map(|p| p.xmp).unwrap_or_default();
         // view state
-        if let Ok(bytes) = std::fs::read(dir.join("view.json"))
-            && let Ok(v) = serde_json::from_slice::<ViewFile>(&bytes)
-        {
+        if let Some(v) = read_json::<ViewFile>(files.as_mut(), "view.json") {
             self.source = v.source;
             self.filter = v.filter;
             self.sort = v.sort;
@@ -142,8 +167,11 @@ impl Session {
             self.selection = Selection::single(*first);
         }
         let presets_written = presets_json(&self.presets);
-        self.media.attach_disk_cache(&dir.join("thumbs"));
-        self.library = Some(Library { dir, journal, report, last_error: None, presets_written });
+        if on_disk {
+            self.media.attach_disk_cache(&dir.join("thumbs"));
+        }
+        let view_written = self.view_json();
+        self.library = Some(Library { dir, on_disk, journal, files, report, last_error: None, presets_written, view_written });
         Ok(&self.library.as_ref().expect("just set").report)
     }
 
@@ -166,7 +194,7 @@ impl Session {
         }
         let presets = presets_json(&self.presets);
         if presets != lib.presets_written {
-            if let Err(e) = write_atomic(&lib.dir, "presets.json", presets.as_bytes()) {
+            if let Err(e) = lib.files.write_atomic("presets.json", presets.as_bytes()) {
                 log::error!("library: presets: {e}");
             } else {
                 lib.presets_written = presets;
@@ -190,20 +218,39 @@ impl Session {
         }
         let _ = self.end_interaction();
         self.persist()?;
-        let view = ViewFile { source: self.source, filter: self.filter.clone(), sort: self.sort, selection: self.selection.clone() };
+        self.save_view();
         let Some(lib) = self.library.as_mut() else { return Ok(()) };
         lib.journal.snapshot(&self.catalog)?;
-        if let Ok(v) = serde_json::to_vec_pretty(&view) {
-            let _ = write_atomic(&lib.dir, "view.json", &v);
-        }
         Ok(())
     }
 
+    fn view_json(&self) -> Vec<u8> {
+        let view = ViewFile { source: self.source, filter: self.filter.clone(), sort: self.sort, selection: self.selection.clone() };
+        serde_json::to_vec_pretty(&view).unwrap_or_default()
+    }
+
+    /// Save the view state (source, filter, sort, selection) if it changed since it was last
+    /// written. Native hosts get this from [`Session::close_library`]; the browser host calls it
+    /// periodically, since a tab can be closed without notice.
+    pub fn save_view(&mut self) {
+        if self.library.is_none() {
+            return;
+        }
+        let v = self.view_json();
+        let Some(lib) = self.library.as_mut() else { return };
+        if v != lib.view_written {
+            match lib.files.write_atomic("view.json", &v) {
+                Ok(()) => lib.view_written = v,
+                Err(e) => log::error!("library: view: {e}"),
+            }
+        }
+    }
+
     /// Save the library preferences (no-op for in-memory sessions).
-    pub fn save_prefs(&self) -> Result<()> {
-        let Some(lib) = &self.library else { return Ok(()) };
+    pub fn save_prefs(&mut self) -> Result<()> {
         let v = serde_json::to_vec_pretty(&PrefsFile { xmp: self.xmp }).unwrap_or_default();
-        write_atomic(&lib.dir, "prefs.json", &v).map_err(|e| EngineError::Other(format!("prefs: {e}")))
+        let Some(lib) = self.library.as_mut() else { return Ok(()) };
+        lib.files.write_atomic("prefs.json", &v).map_err(|e| EngineError::Other(format!("prefs: {e}")))
     }
 
     /// Compact the log into a snapshot now.