fix(flutter): player updates on track change + kebab artist nav + Start radio

Three issues, all related to the player surface:

1. Player UI didn't update on track change. audio_handler's
   _onCurrentIndexChanged only kicked off the cover load — it never
   pushed the new MediaItem onto the mediaItem stream. Title/artist/
   cover stayed pinned to whatever setQueueFromTracks(initialIndex:)
   set on first play. Now the listener pushes queue[idx] when the
   index changes.

2. Player kebab "Go to artist" 404'd while the same item from
   MostPlayed worked. Same TrackActionsSheet for both, but the
   player's _trackRefFromMediaItem was hardcoding artistId: ''
   because audio_handler's _toMediaItem never stashed it in extras.
   Stash artist_id alongside album_id; player_bar +
   now_playing_screen read it back. Both kebabs now navigate.

3. "Start radio" didn't exist on Flutter even though the server has
   /api/radio?seed_track=<id>. New RadioApi (lib/api/endpoints/
   radio.dart) wraps the endpoint; PlayerActions.startRadio(trackId)
   fetches + plays the result via the existing playTracks path.
   New menu item between "Add to playlist" and the divider above
   "Go to album", calls startRadio with a snackbar error fallback.

flutter_client/lib/player/player_provider.dart

@@ -1,6 +1,7 @@
 import 'package:audio_service/audio_service.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../api/endpoints/radio.dart';
 import '../auth/auth_provider.dart';
 import '../cache/audio_cache_manager.dart';
 import '../library/library_providers.dart' show dioProvider;
@@ -102,6 +103,15 @@ class PlayerActions {
   Future<void> setVolume(double v) async {
     await _ref.read(audioHandlerProvider).setVolume(v);
   }
+
+  /// Fetches /api/radio?seed_track=<id> and starts playing the
+  /// returned track list (seed at index 0 + recommended picks).
+  Future<void> startRadio(String trackId) async {
+    final dio = await _ref.read(dioProvider.future);
+    final tracks = await RadioApi(dio).seedTrack(trackId);
+    if (tracks.isEmpty) return;
+    await playTracks(tracks);
+  }
 }
 
 final playerActionsProvider = Provider<PlayerActions>((ref) => PlayerActions(ref));