fix(sync): harden sync CLI, IPC, and UI paths from CodeRabbit review

- reject option-like tokens as flag values (--snapshot --force wrote a
  file named --force); --flag=-value still works
- PowerShell remote quoting uses single-quoted literals so $() in a
  quoted path cannot expand
- sync-hosts.json written via temp file + rename; a crash mid-write
  truncated it and the reader's corrupt-fallback dropped every host
- cancelled sync child escalates SIGTERM -> SIGKILL after 5s grace
- NDJSON progress events validated field-by-field before casting
- snapshot filenames include milliseconds to avoid same-second overwrite
- syncAutoScheduler.stop() wired into will-quit cleanup
- sync --ui exclusivity also rejects --make-temp/--remove-temp/--json
- document --sync-window in app help; group --make-temp/--remove-temp
  under modes in sync usage

src/main/runtime/app-lifecycle-main-cleanup.test.ts

@@ -19,6 +19,7 @@ test('cleanup deps builder returns handlers that guard optional runtime objects'
     unregisterAllGlobalShortcuts: () => calls.push('unregister-shortcuts'),
     stopSubtitleWebsocket: () => calls.push('stop-ws'),
     stopTexthookerService: () => calls.push('stop-texthooker'),
+    stopSyncAutoScheduler: () => calls.push('stop-sync-auto-scheduler'),
     clearWindowsVisibleOverlayForegroundPollLoop: () =>
       calls.push('clear-windows-visible-overlay-foreground-poll-loop'),
     clearLinuxMpvFullscreenOverlayRefreshTimeouts: () =>
@@ -113,6 +114,7 @@ test('cleanup deps builder skips destroyed yomitan window', () => {
     unregisterAllGlobalShortcuts: () => {},
     stopSubtitleWebsocket: () => {},
     stopTexthookerService: () => {},
+    stopSyncAutoScheduler: () => {},
     clearWindowsVisibleOverlayForegroundPollLoop: () => {},
     clearLinuxMpvFullscreenOverlayRefreshTimeouts: () => {},
     getMainOverlayWindow: () => ({
@@ -173,6 +175,7 @@ test('cleanup deps builder skips global shortcut cleanup before app ready', () =
     },
     stopSubtitleWebsocket: () => {},
     stopTexthookerService: () => {},
+    stopSyncAutoScheduler: () => {},
     clearWindowsVisibleOverlayForegroundPollLoop: () => {},
     clearLinuxMpvFullscreenOverlayRefreshTimeouts: () => {},
     getMainOverlayWindow: () => null,