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.ts

@@ -3951,6 +3951,7 @@ const {
       annotationSubtitleWsService.stop();
     },
     stopTexthookerService: () => texthookerService.stop(),
+    stopSyncAutoScheduler: () => syncAutoScheduler.stop(),
     clearWindowsVisibleOverlayForegroundPollLoop: () =>
       clearWindowsVisibleOverlayForegroundPollLoop(),
     clearLinuxMpvFullscreenOverlayRefreshTimeouts: () => {