Protocol 13: Up next preloads, so switching to it is instant

Every app gets the first Up next item ready itself (own Jellyfin plan,
direct links, guests via /j when state.relayNext says the server's
login can pass it on) and opens it in a hidden, muted, paused second
player; the swap promotes that player instead of opening cold, so its
buffer counts at once. Reports go out on open and when the buffer
reaches the gate's need. SwitchTrace times each phase. Measured in the
e2e harness: 1.5-3 s before, 35-103 ms with a preload. Relayed items
still open cold for everyone but their source (ROADMAP).

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

packages/server/lib/src/server_hub.dart

@@ -30,8 +30,16 @@ class ServerHub {
       : engine = engine ?? SyncEngine();
 
   /// The current media is a Jellyfin ref on the server [jellyfin] logs in to.
-  bool get relayAvailable {
-    final ref = session.media?.jellyfin;
+  bool get relayAvailable => _onLogin(session.media?.jellyfin);
+
+  /// Up next's first item is a Jellyfin ref on that server: `/j` serves it
+  /// too, so viewers without access get it ready.
+  bool get relayNextAvailable => _onLogin(nextJellyfinRef);
+
+  /// Up next's first item's Jellyfin ref (null: none, or not a ref).
+  JellyfinRef? get nextJellyfinRef => engine.queue.items.firstOrNull?.media.jellyfin;
+
+  bool _onLogin(JellyfinRef? ref) {
     final cred = jellyfin;
     return ref != null && cred != null && cred.serverId.isNotEmpty && cred.serverId == ref.serverId;
   }
@@ -53,7 +61,7 @@ class ServerHub {
         : '$head: $why';
   }
 
-  StateBroadcast stateBroadcast() => engine.broadcast(relay: relayAvailable);
+  StateBroadcast stateBroadcast() => engine.broadcast(relay: relayAvailable, relayNext: relayNextAvailable);
 
   /// The relay of the current media (`/r/...`), if it is relayed.
   ActiveRelay? relay;