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/API.csproj

@@ -1,18 +1,14 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
 
-  <PropertyGroup>
-    <TargetFramework>net9.0</TargetFramework>
-    <Nullable>enable</Nullable>
-    <ImplicitUsings>enable</ImplicitUsings>
-  </PropertyGroup>
+    <PropertyGroup>
+        <TargetFramework>net9.0</TargetFramework>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+    </PropertyGroup>
 
-  <ItemGroup>
-    <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="9.0.12" />
-    <PackageReference Include="Swashbuckle.AspNetCore" Version="7.2.0" />
-  </ItemGroup>
-
-  <ItemGroup>
-    <Folder Include="Repository\" />
-  </ItemGroup>
+    <ItemGroup>
+        <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="9.0.12"/>
+        <PackageReference Include="Swashbuckle.AspNetCore" Version="7.2.0"/>
+    </ItemGroup>
 
 </Project>