feat: persist worker-pause state across restarts

Worker pause states can optionally survive app restarts. A new toggle in
Settings saves the current pause map to settings/worker_pauses.json; on
startup lib.rs restores it before workers are spawned. Backend: new
snapshot/replace helpers in indexer.rs, persist functions in commands.rs
(get/set_worker_pauses_persist). Frontend: workerPausesPersist store
field, load/setWorkerPausesPersist actions, toggle in SettingsModal.

src/App.tsx

@@ -27,6 +27,7 @@ export default function App() {
   const loadAlbums = useGalleryStore((state) => state.loadAlbums);
   const loadMutedFolderIds = useGalleryStore((state) => state.loadMutedFolderIds);
   const loadNotificationsPaused = useGalleryStore((state) => state.loadNotificationsPaused);
+  const loadWorkerPausesPersist = useGalleryStore((state) => state.loadWorkerPausesPersist);
   const subscribeToProgress = useGalleryStore((state) => state.subscribeToProgress);
   const loadAppVersion = useGalleryStore((state) => state.loadAppVersion);
   const checkForUpdates = useGalleryStore((state) => state.checkForUpdates);
@@ -39,6 +40,7 @@ export default function App() {
     void initializeNotifications();
     void loadMutedFolderIds();
     void loadNotificationsPaused();
+    void loadWorkerPausesPersist();
     void loadFfmpegStatus();
     void loadOnboardingCompleted();
     // Load the app version first so the What's New toast/modal (which read