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/Repository/Game/GameRepository.cs

@@ -0,0 +1,5 @@
+namespace API.Repository.Game;
+
+public class GameRepository : IGameRepository
+{
+}