Fix up #247: route main's other file access through the checked io layer

design.browseTheme reads its path with io::read_file, and File › Info sizes the file with
io::file_len (which checks the read root), so commands_do_not_touch_the_file_system_themselves
passes. spell.rs is allow-listed beside recovery.rs: it touches only the app's own dictionary
folders. main's render_slide MCP tool refuses a `path` argument (#371), so the root tests
render to a file through call_app ui.render instead.

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

crates/engine/src/cmd/file.rs

@@ -476,7 +476,7 @@ fn info(s: &mut Session, _p: &Value) -> Result<Value> {
     let st = s.doc()?;
     let doc = &st.doc;
     #[cfg(not(target_arch = "wasm32"))]
-    let bytes = st.path.as_ref().and_then(|p| std::fs::metadata(p).ok()).map(|m| m.len());
+    let bytes = st.path.as_deref().and_then(crate::io::file_len);
     #[cfg(target_arch = "wasm32")]
     let bytes: Option<u64> = None;
     Ok(json!({