feat(17-01): add TrackMetadataChanged handler and remove selection gate on Track Details

- LibraryStore now listens for TrackMetadataChanged and invalidates all caches
- Track Details context menu item visible for any right-clicked track in all 4 views
- Removed selectionCount === 1 gate from track-list, queue-panel, cover-grid, playlist-details

frontend/src/store/library-store.ts

@@ -82,6 +82,9 @@ class LibraryStore {
             this.changeGen++;
             this.notify();
         });
+        EventsOn(Events.TrackMetadataChanged, () => {
+            this.invalidate();
+        });
 
         this.loadCoverSize();
         this.deferEagerFetch();