Lobby: include HostUserId in GameStateDto and let client derive host status; update tests

SlipItIn.Shared/DTOs/GameStateDto.cs

@@ -6,6 +6,8 @@ public class GameStateDto
     public string LobbyCode { get; set; } = string.Empty;
     public string Status { get; set; } = string.Empty;
     public List<PlayerInfoDto> Players { get; set; } = [];
+    // Host user id to let clients determine whether the current user is the host
+    public int HostUserId { get; set; }
     public int CurrentRound { get; set; }
     public int RoundTimeRemaining { get; set; }
 }