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/shared/widgets/track_actions/track_actions_sheet.dart

@@ -117,6 +117,23 @@ class TrackActionsSheet extends ConsumerWidget {
                 }
               },
             ),
+            _MenuItem(
+              key: const Key('track_actions_start_radio'),
+              icon: Icons.radio,
+              label: 'Start radio',
+              onTap: () async {
+                Navigator.pop(context);
+                try {
+                  await ref.read(playerActionsProvider).startRadio(track.id);
+                } catch (e) {
+                  if (context.mounted) {
+                    ScaffoldMessenger.of(context).showSnackBar(
+                      SnackBar(content: Text("Couldn't start radio: $e")),
+                    );
+                  }
+                }
+              },
+            ),
             const _Divider(),
             _MenuItem(
               key: const Key('track_actions_go_to_album'),