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/api/endpoints/radio.dart

@@ -0,0 +1,27 @@
+import 'package:dio/dio.dart';
+
+import '../../models/track.dart';
+
+/// GET /api/radio?seed_track=<uuid>&limit=<int>. Returns the seed at
+/// index 0 followed by up to `limit-1` weighted-shuffle picks scored
+/// by the server's recommendation engine. The shape matches what
+/// `playerActions.playTracks` expects, so a radio start is just one
+/// fetch + one playTracks call.
+class RadioApi {
+  RadioApi(this._dio);
+  final Dio _dio;
+
+  Future<List<TrackRef>> seedTrack(String trackId, {int? limit}) async {
+    final r = await _dio.get<Map<String, dynamic>>(
+      '/api/radio',
+      queryParameters: {
+        'seed_track': trackId,
+        if (limit != null) 'limit': limit,
+      },
+    );
+    final raw = (r.data?['tracks'] as List?) ?? const [];
+    return raw
+        .map((e) => TrackRef.fromJson((e as Map).cast<String, dynamic>()))
+        .toList(growable: false);
+  }
+}