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/audio_handler.dart

@@ -165,19 +165,36 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
     queue.add([...queue.value, item]);
   }
 
-  MediaItem _toMediaItem(TrackRef t) => MediaItem(
-        id: t.id,
-        title: t.title,
-        artist: t.artistName,
-        album: t.albumTitle,
-        duration: Duration(seconds: t.durationSec),
-        // Stash album_id in extras so _loadArtForCurrentItem can pull it
-        // back without re-walking the track list.
-        extras: t.albumId.isEmpty ? null : {'album_id': t.albumId},
-      );
+  MediaItem _toMediaItem(TrackRef t) {
+    // Stash album_id + artist_id in extras so widgets reconstructing
+    // a TrackRef from the MediaItem (player kebab → "Go to artist",
+    // "Go to album") have the IDs they need to navigate. Earlier code
+    // only carried album_id which left "Go to artist" pushing
+    // /artists/ (empty id, route 404).
+    final extras = <String, dynamic>{};
+    if (t.albumId.isNotEmpty) extras['album_id'] = t.albumId;
+    if (t.artistId.isNotEmpty) extras['artist_id'] = t.artistId;
+    return MediaItem(
+      id: t.id,
+      title: t.title,
+      artist: t.artistName,
+      album: t.albumTitle,
+      duration: Duration(seconds: t.durationSec),
+      extras: extras.isEmpty ? null : extras,
+    );
+  }
 
   void _onCurrentIndexChanged(int? idx) {
     if (idx == null) return;
+    // Push the new track's MediaItem onto the mediaItem stream so
+    // the player UI rebuilds with the new title/artist/album/cover.
+    // Without this, the bar and full player stayed pinned to whichever
+    // track was passed via setQueueFromTracks(initialIndex:) regardless
+    // of skip/auto-advance.
+    final items = queue.value;
+    if (idx >= 0 && idx < items.length) {
+      mediaItem.add(items[idx]);
+    }
     unawaited(_loadArtForCurrentItem());
   }
 

flutter_client/lib/player/now_playing_screen.dart

@@ -235,7 +235,7 @@ class _TitleRow extends StatelessWidget {
           title: media.title,
           albumId: (media.extras?['album_id'] as String?) ?? '',
           albumTitle: media.album ?? '',
-          artistId: '',
+          artistId: (media.extras?['artist_id'] as String?) ?? '',
           artistName: media.artist ?? '',
           durationSec: media.duration?.inSeconds ?? 0,
           streamUrl: '',

flutter_client/lib/player/player_bar.dart

@@ -286,7 +286,9 @@ TrackRef _trackRefFromMediaItem(MediaItem media) => TrackRef(
       title: media.title,
       albumId: (media.extras?['album_id'] as String?) ?? '',
       albumTitle: media.album ?? '',
-      artistId: '',
+      // artist_id is stashed in extras by audio_handler — without it
+      // the kebab's "Go to artist" pushes /artists/ (empty id) and 404s.
+      artistId: (media.extras?['artist_id'] as String?) ?? '',
       artistName: media.artist ?? '',
       durationSec: media.duration?.inSeconds ?? 0,
       streamUrl: '',

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));