fix(watch-together): re-host an abandoned room code instead of joining it

A room whose peers have all left is a code nobody is using, but the relay
kept it bound to the creator's reconnect capability and rejected every
other create with room_exists. The app compounded it: enterRoom only
promoted to host on room_not_found, so tapping a recent code landed the
user in the retained room as a guest of a host that was never coming
back, until the cleanup sweep finally dropped the room.

Create now replaces a room with no connected peers, and enterRoom hosts
the code when its probe join finds an empty room. An occupied room still
rejects create, including from its previous owner, and a host that is
merely disconnected still reclaims its peer ID through join with the
matching token.

lib/watch_together/providers/watch_together_provider.dart

@@ -438,8 +438,8 @@ class WatchTogetherProvider with ChangeNotifier {
     }
   }
 
-  /// Enter a room by code — joins any reserved room and creates only when the
-  /// relay reports that no room reservation exists.
+  /// Enter a room by code — joins a room that still has someone in it and
+  /// hosts the code otherwise.
   ///
   /// Returns `true` if the user became the host.
   Future<bool> enterRoom(
@@ -453,13 +453,19 @@ class WatchTogetherProvider with ChangeNotifier {
     final probe = _peerServiceFactory(endpoint: relayEndpoint);
     var shouldBeHost = false;
     try {
+      var probeJoined = false;
       try {
         await probe.joinSession(sessionId);
+        probeJoined = true;
       } on PeerError catch (error) {
         if (error.serverCode != RelayProtocol.roomNotFoundCode) rethrow;
-        shouldBeHost = true;
       }
-      if (!shouldBeHost) {
+      // A room the relay still holds but nobody is connected to is an
+      // abandoned code, not a session: its declared host is gone and the
+      // reservation only survives until the next cleanup sweep. Take the code
+      // over instead of waiting on a host that is never coming back.
+      shouldBeHost = !probeJoined || probe.connectedPeers.isEmpty;
+      if (probeJoined) {
         await probe.releaseSession();
       }
     } finally {