feat: implement ListenBrainz integration (Stage 2)

Add ListenBrainz scrobbling support with:
- Client module for ListenBrainz API
- Token-based authentication (user provides token from listenbrainz.org)
- Now-playing submission support
- Scrobble submission with MusicBrainz enrichment
- SQLite-based offline queue with TTL
- Credential persistence across sessions
- Settings UI for connection management

API integration uses Stage 1 MusicBrainz data to enrich scrobbles with
recording MBID, release MBID, and artist MBIDs when available.

cc

src-tauri/src/commands/mod.rs

@@ -9,6 +9,7 @@ pub mod cache;
 pub mod credits;
 pub mod favorites;
 pub mod lastfm;
+pub mod listenbrainz;
 pub mod musicbrainz;
 pub mod notification;
 pub mod playback;
@@ -27,6 +28,7 @@ pub use cache::*;
 pub use credits::*;
 pub use favorites::*;
 pub use lastfm::*;
+pub use listenbrainz::*;
 pub use musicbrainz::*;
 pub use notification::*;
 pub use playback::*;