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>

ui/src/app/format.ts

@@ -23,6 +23,25 @@ export function formatDate(epochMs: number): string {
   return DATE_FORMAT.format(new Date(epochMs));
 }
 
+/** bytes/sec -> "14.2 MB/s" or "113.8 Mbps" depending on the configured unit. */
+export function formatSpeed(bytesPerSec: number, unit: 'MBps' | 'Mbps'): string {
+  if (unit === 'Mbps') {
+    return `${((bytesPerSec * 8) / 1_000_000).toFixed(1)} Mbps`;
+  }
+  return `${formatSize(bytesPerSec)}/s`;
+}
+
+/** Seconds -> "0:42" / "3:07" / "1:02:15". */
+export function formatEta(seconds: number): string {
+  if (!Number.isFinite(seconds) || seconds < 0) return '';
+  const s = Math.round(seconds);
+  const h = Math.floor(s / 3600);
+  const m = Math.floor((s % 3600) / 60);
+  const sec = s % 60;
+  const mmss = `${m}:${String(sec).padStart(2, '0')}`;
+  return h ? `${h}:${String(m).padStart(2, '0')}:${String(sec).padStart(2, '0')}` : mmss;
+}
+
 /** POSIX-style join for UI-composed paths (both panels use "/" on macOS + remote). */
 export function joinPath(dir: string, name: string): string {
   return dir.endsWith('/') ? dir + name : `${dir}/${name}`;