Player: now-playing title with cover, start banner, one status pill; media art for everyone

The swapper sends the media's poster and banner once (`art`, Jellyfin
images or AniList by parsed name); state carries their hashes and every
member fetches them from /i/ like avatars. state.media.swapBy names who
put the media up. uosc and the standard bar show cover, title and
episode; a banner card shows before playback starts. The top-left health
pill and the waiting pill become one frosted status pill in plain words.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/sync/sync_client.dart

@@ -250,6 +250,11 @@ class SyncClient {
     _send(ProfileIntent(name: name, avatar: avatar).toJson());
   }
 
+  /// Art for media [mediaId] (base64 picture bytes; null = none).
+  void sendArt(int mediaId, {String? poster, String? banner}) {
+    _send(ArtMessage(mediaId: mediaId, poster: poster, banner: banner).toJson());
+  }
+
   void sendKick(String target, {String reason = ''}) {
     _send(KickRequest(target: target, reason: reason).toJson());
   }