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/test/avatar_cache_test.dart

@@ -56,4 +56,35 @@ void main() {
       await server.stop();
     }
   });
+
+  test('media art arrives by hash over /i/; a swap clears it', () async {
+    final server = SyncServer(const AppConfig(port: 0, secret: 's', name: 'art'));
+    await server.start();
+    final client = SyncClient();
+    final cache = ArtCache(client);
+    final poster = Uint8List.fromList([0xFF, 0xD8, 0xFF, ...List<int>.generate(197, (i) => i % 256)]);
+    try {
+      await client.connect(
+        wsUri: Uri.parse('ws://127.0.0.1:${server.boundPort}/sync'),
+        secret: 's',
+        name: 'alice',
+      );
+      client.sendSwap(const HelloMedia(name: 'Show - 01.mkv', size: 0, duration: 0, url: 'http://example.com/a.mkv'));
+      await _poll(() => client.lastState?.media?['swapBy'] == client.memberId, 'swap');
+      client.sendArt(client.lastState!.mediaId, poster: base64Encode(poster));
+      await _poll(() => ArtCache.artHashesOf(client.lastState!).isNotEmpty, 'art in state');
+      final hash = ArtCache.artHashesOf(client.lastState!).single;
+      expect(hash, avatarHashOf(poster));
+      await _poll(() => cache.bytesFor(hash) != null, 'art bytes');
+      expect(cache.bytesFor(hash), poster);
+
+      client.sendSwap(const HelloMedia(name: 'Other.mkv', size: 0, duration: 0, url: 'http://example.com/b.mkv'));
+      await _poll(() => client.lastState?.media?['name'] == 'Other.mkv', 'second swap');
+      expect(ArtCache.artHashesOf(client.lastState!), isEmpty);
+    } finally {
+      cache.dispose();
+      await client.dispose();
+      await server.stop();
+    }
+  });
 }