Refactor and clean up codebase:

- Introduced `IGameRepository` interface and its implementation.
- Transitioned namespaces to `file-scoped` for consistency.
- Simplified class definitions, e.g., `Game` and `SixDigitInt`.
- Restructured `GameField` logic for improved readability and functionality.
- Fixed formatting issues in `API.csproj` and added a `using` directive in `Program.cs`.

API/Models/Game/Game.cs

@@ -1,26 +1,19 @@
 using API.Models.DataClasses;
 
-namespace API.Models.Game
+namespace API.Models.Game;
+
+public enum GameState
 {
-    public enum GameState
-    {
-        Lobby,
-        Running,
-        Ended
-    }
-
-    public class Game
-    {
-        public string Id { get; init; } = Guid.NewGuid().ToString();
-        public SixDigitInt GameCode { get; }
-        public string?[] PlayerConnectionIds { get; set; } = new string?[2];
-        public GameState State { get; private set; } = GameState.Lobby;
-        public GameField Field { get; }
-
-        public Game(Coordinates gFs, SixDigitInt gameCode)
-        {
-            Field = new GameField(gFs);
-            GameCode = gameCode;
-        }
-    }
+    Lobby,
+    Running,
+    Ended
 }
+
+public class Game(Coordinates gFs, SixDigitInt gameCode)
+{
+    public string Id { get; init; } = Guid.NewGuid().ToString();
+    public SixDigitInt GameCode { get; } = gameCode;
+    public string?[] PlayerConnectionIds { get; set; } = new string?[2];
+    public GameState State { get; private set; } = GameState.Lobby;
+    public GameField Field { get; } = new(gFs);
+}