✨ WebRTC signaling heartbeat

lib/models/chat.dart

@@ -152,6 +152,7 @@ sealed class CallParticipant with _$CallParticipant {
     required String accountId,
     @Default(null) SnAccount? account,
     required DateTime joinedAt,
+    @Default(false) bool isLocal,
   }) = _CallParticipant;
 
   factory CallParticipant.fromJson(Map<String, dynamic> json) =>