feat(quick-17): create playlist-details subpage component

- New playlist-details component with header (back button, playlist icon, title, track count)
- Full track list with all interactions: select, play, context menu, drag, phantom handling
- Drop target support for adding tracks from other views
- Search filtering for tracks within the detail view
- Navigation routing in index.ts for playlist-details view
- Added playlist-details to SEARCHABLE_VIEWS in search-store

frontend/src/store/search-store.ts

@@ -1,5 +1,5 @@
 /** Searchable views that respond to the global search term. */
-const SEARCHABLE_VIEWS = new Set(['tracks', 'albums', 'playlists', 'artists', 'genres']);
+const SEARCHABLE_VIEWS = new Set(['tracks', 'albums', 'playlists', 'playlist-details', 'artists', 'genres']);
 
 type Subscriber = () => void;