Add turn tracking and enforce turn order

Introduce CurrentTurn to Game (default 1) and expose it via GameInformationDto. Update GameManager to include CurrentTurn in the GameInformation payload, validate that a player can only Drop when it is their turn, and toggle CurrentTurn between 1 and 2 after a successful move. Also use a safe fallback (0) when game is null.

API/Models/Game/Game.cs

@@ -14,6 +14,7 @@ public class Game(Coordinates gFs, SixDigitInt gameCode)
     public string Id { get; init; } = Guid.NewGuid().ToString();
     public SixDigitInt GameCode { get; } = gameCode;
     public List<Player> Players { get; set; } = new();
+    public int CurrentTurn { get; set; } = 1;
     public GameState State { get; private set; } = GameState.Lobby;
     public GameField Field { get; } = new(gFs);