feat(watcher): adaptive filesystem watchdog with zero CPU when idle

Monitors all registered folders using OS-native events (ReadDirectoryChangesW
on Windows) so new, modified, and deleted files are reflected in the gallery
automatically without manual reindexing.

Key design points:
- Adaptive blocking: recv() when no events pending (zero CPU), switches to
  recv_timeout(earliest_deadline) only when debounce timers are running — wakes
  exactly when the soonest event is ready, no busy-polling
- 500 ms per-path debounce coalesces rapid OS event bursts into one action
- Change detection preserved: build_record skips upsert if file_size + mtime
  unchanged, preventing spurious thumbnail/embedding re-queues and avoiding
  clobbering of existing metadata like thumbnail_path
- Access events (reads) filtered out; only Create/Modify/Remove trigger work
- Deletion path: emits watcher-deleted event with image IDs; frontend removes
  those images from state and clears selectedImage if it was deleted

WatcherHandle stored in app state; add_folder / remove_folder / update_folder_path
commands keep the watched path set in sync with the DB.

src-tauri/src/lib.rs

@@ -74,8 +74,11 @@ pub fn run() {
             // indexer::start_caption_worker(app.handle().clone(), pool.clone(), app_dir.clone());
             indexer::start_tagging_worker(app.handle().clone(), pool.clone(), app_dir.clone());
 
+            let watcher_handle = indexer::start_watcher(app.handle().clone(), pool.clone());
+
             app.manage(pool);
             app.manage(media_tools);
+            app.manage(watcher_handle);
 
             Ok(())
         })