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/DataClasses/SixDigitInt.cs

@@ -1,20 +1,25 @@
-namespace API.Models.DataClasses
+namespace API.Models.DataClasses;
+
+public readonly record struct SixDigitInt
 {
-    public readonly record struct SixDigitInt
+    public SixDigitInt(int value)
     {
-        public int Value { get; }
+        if (value < 0 || value > 999999)
+            throw new ArgumentOutOfRangeException(nameof(value),
+                "Wert muss zwischen 0 und 999999 liegen.");
 
-        public SixDigitInt(int value)
-        {
-            if (value < 0 || value > 999999)
-                throw new ArgumentOutOfRangeException(nameof(value),
-                    "Wert muss zwischen 0 und 999999 liegen.");
-
-            Value = value;
-        }
-
-        public override string ToString() => Value.ToString("D6");
-
-        public static implicit operator int(SixDigitInt v) => v.Value;
+        Value = value;
     }
-}
+
+    public int Value { get; }
+
+    public override string ToString()
+    {
+        return Value.ToString("D6");
+    }
+
+    public static implicit operator int(SixDigitInt v)
+    {
+        return v.Value;
+    }
+}