Add playlist and favorites commands, update sidebar

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

src-tauri/src/commands/mod.rs

@@ -3,11 +3,15 @@
 //! Exposes backend functionality to the frontend via IPC
 
 pub mod auth;
+pub mod favorites;
 pub mod playback;
+pub mod playlist;
 pub mod queue;
 pub mod search;
 
 pub use auth::*;
+pub use favorites::*;
 pub use playback::*;
+pub use playlist::*;
 pub use queue::*;
 pub use search::*;