M4.36: Background Save and Export write a snapshot off the UI thread with status bar progress

- With Preferences > File Handling > Enable Background Save / Export on, File >
  Save, Save As, Save a Copy and exports encode and write a snapshot of the
  document on a worker thread; the call returns at once ({path, background}).
- The status bar shows a spinner and what is being written; ui.inspect lists
  the running jobs under `background`.
- The document takes the path and counts as saved as of the snapshot once the
  file is written: edits made meanwhile keep it modified.
- Quit and Close wait for running saves; the Save Changes dialog saves at once.
- The host gives a thread-safe writer (Services::write_shared, atomic on the
  desktop); without one (web, tests) everything runs at once as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/vectorcraft/src/main.rs

@@ -138,6 +138,11 @@ fn reveal_command(path: &str) -> std::process::Command {
     c
 }
 
+/// Write a file the safe way: a failed write keeps the old file ([`fileio::write_atomic`]).
+fn write_file(path: &str, bytes: &[u8]) -> Result<(), String> {
+    fileio::write_atomic(std::path::Path::new(path), bytes).map_err(|e| e.to_string())
+}
+
 fn services() -> Services {
     Services {
         pick_open: Some(Box::new(|pick: &FilePick| file_dialog(pick).pick_file().map(|p| p.to_string_lossy().to_string()))),
@@ -158,7 +163,9 @@ fn services() -> Services {
             file_dialog(pick).save_file().map(|p| p.to_string_lossy().to_string())
         })),
         read: Some(Box::new(|p: &str| std::fs::read(p).map_err(|e| e.to_string()))),
-        write: Some(Box::new(|p: &str, b: &[u8]| fileio::write_atomic(std::path::Path::new(p), b).map_err(|e| e.to_string()))),
+        write: Some(Box::new(write_file)),
+        // Background Save and Export write from a worker thread.
+        write_shared: Some(std::sync::Arc::new(write_file)),
         // Every format Copy offers and Paste reads (menu-bar Paste never sees egui's Paste event).
         system_clipboard: Some(clipboard::system_clipboard()),
         // Help → Discord / website / GitHub, the Discord button, About and Home links.