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/app.ts

@@ -4,6 +4,7 @@ import { BRANDING } from '@shared/branding';
 import { ConnectionManager } from './connection-manager/connection-manager';
 import { ChmodDialog } from './dialogs/chmod-dialog';
 import { ConfirmDialog } from './dialogs/confirm-dialog';
+import { OverwriteDialog } from './dialogs/overwrite-dialog';
 import { PromptDialog } from './dialogs/prompt-dialog';
 import { DialogsService } from './dialogs.service';
 import { FilePanel } from './file-panel/file-panel';
@@ -24,6 +25,7 @@ import { TransferBar } from './transfer-bar/transfer-bar';
     PromptDialog,
     ConfirmDialog,
     ChmodDialog,
+    OverwriteDialog,
     TransferBar,
     ConnectionManager,
   ],