v1.2: overwrite confirm, transfer speed/ETA, settings + themes, fused macOS titlebar

- Overwrite confirmation: name-collision detection before enqueue; dialog shows
  source vs destination (size, mtime, 'newer' badge) with per-item Skip/Replace
  and batch Skip all / Replace all; type mismatches auto-skip with a toast;
  cancel aborts the whole operation
- Transfer speed + ETA: UI computes smoothed bytes/sec from progress deltas
  (callback bus makes deltas meaningful); shown in the expanded queue rows and
  the status-bar summary; unit follows the speed-unit setting
- Settings: bun-side SettingsStore (settings.json, 0600, atomic) + get/save RPC;
  SettingsService; gear popover on the tab row (theme segmented control, speed
  unit); persisted across restarts
- Light theme: data-theme on <html> drives token overrides; absent = follow OS
- macOS fused titlebar: hiddenInset + trafficLightOffset, tab row becomes the
  draggable titlebar (app-region drag/no-drag classes); other platforms keep a
  separate titlebar (gated by a platform value injected via window preload)
- README: known-issues note for the upstream Bun/Electrobun FFI crash

Verified: tsc + ui build clean; SettingsStore roundtrip + 0600 headless;
app boots and the UI fetches settings over RPC at startup.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -17,7 +17,14 @@ remote server over SSH/SFTP, so you can move files **local↔remote**,
   sessions stay connected and transfers keep running in background tabs.
 - Per-panel action toolbar with a direction-explicit copy button, plus a
   VSCode-style status bar showing live transfer progress (click to expand the
-  transfer queue).
+  transfer queue with per-file speed and ETA).
+- **Overwrite confirmation** on name collisions, with a source-vs-destination
+  comparison (size, modified date, which is newer) and batch replace-all /
+  skip-all.
+- **Settings** (gear, top-right): light / dark / system theme and speed units
+  (MB/s or Mbps), persisted across restarts.
+- On macOS the tab bar is fused with the window titlebar; other platforms keep
+  a standard titlebar.
 - Browse, sort (name/size/modified, folders first), keyboard navigation.
 - Create folder / file, rename, delete (recursive), change permissions (chmod).
 - Upload, download, and remote→remote copy — files **and** whole folders —
@@ -129,6 +136,17 @@ Saved connections live in a plain JSON file in the OS app-data directory
 
 (On Macs, function keys may need `fn` — every action also has a toolbar button.)
 
+## Known issues
+
+- **Rare native crash under load.** A long-running session (multiple remote
+  connections, extended uptime) can hit a Bun panic — a segfault inside Bun
+  1.3.13's FFI thread-safe callbacks executing Electrobun's native→worker
+  bridge (`FFI_Callback_threadsafe_call` → `web_worker.spin` in the decoded
+  crash report). This is an upstream Bun × Electrobun bug, not app code; a
+  restart recovers cleanly and in-flight work isn't corrupted. Expected to
+  resolve when Electrobun bumps its pinned Bun (currently 1.3.13; 1.3.14+
+  available). Bun's own crash reporter link is printed to the console on panic.
+
 ## Not in v1
 
 Drag-and-drop, file preview/edit, transfer resume, symlink following