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>

packages/server/lib/src/sync_server.dart

@@ -65,6 +65,7 @@ class SyncServer {
     mountSessionMediaRoute(_app, sessions);
     mountRelayRoutes(_app, sessions, () => hub.relay);
     mountAvatarRoute(_app, hub);
+    mountArtRoute(_app, hub);
     mountJellyfinRelayRoute(
         _app, sessions, () => hub.jellyfin, () => _relayClient ??= HttpClient());
     // advertiseHost is only an explicit override; otherwise echo what the client dialled.