feat(sync-ui): defer app quit until async cleanup resolves

- check-host participates in active-run coordination and can be cancelled
- shutdown() cancels and awaits the active launcher run on quit
- onWillQuit calls preventDefault and re-triggers quit once cleanup settles
- Snapshot mode awaits tracker quiescent before writing output
- Auto-scheduler catches synchronous triggerHostSync failures
- SCP endpoint accepts Windows absolute paths; cmd.exe rejects % in quoted values
- runAppCommand unified (inherit vs pipe stdio) in launcher/mpv.ts
- Docs: add --check, --json, --ui, --sync-cli, --make-temp/--remove-temp examples

src/main/startup-lifecycle.ts

@@ -14,7 +14,7 @@ export interface AppLifecycleRuntimeRunnerParams {
   logNoRunningInstance: () => void;
   startControlServer?: (handleArgv: (argv: string[]) => void) => (() => void) | void;
   onReady: () => Promise<void>;
-  onWillQuitCleanup: () => void;
+  onWillQuitCleanup: () => void | Promise<void>;
   shouldRestoreWindowsOnActivate: () => boolean;
   restoreWindowsOnActivate: () => void;
   shouldQuitOnWindowAllClosed: () => boolean;