Backend:
- Add playlist.rs commands: get_user_playlists, get_playlist, search_playlists
- Add favorites.rs commands: get_favorites, add_favorite, remove_favorite
- Add API client methods: search_playlists, add_favorite, remove_favorite
Frontend:
- Remove Radio and New from sidebar (no API support)
- Sidebar now loads and displays user playlists dynamically
- Add playlist and favorites views (placeholder for now)
- Pass selectedPlaylistId and onPlaylistSelect to Sidebar