refactor: implement singleton pattern for key managers and improve resource access

scripts/UI/Credits.cs

@@ -1,24 +1,18 @@
 using Godot;
-using Mr.BrickAdventures;
 using Mr.BrickAdventures.Autoloads;
 
 namespace Mr.BrickAdventures.scripts.UI;
 
 public partial class Credits : Control
 {
-    private UIManager _uiManager;
-
-    public override void _Ready()
-    {
-        _uiManager = GetNode<UIManager>(Constants.UIManagerPath);
-    }
+    private UIManager UIManager => UIManager.Instance;
 
     public override void _UnhandledInput(InputEvent @event)
     {
         if (!@event.IsActionPressed("ui_cancel")) return;
-        if (_uiManager != null && _uiManager.IsScreenOnTop(this))
+        if (UIManager != null && UIManager.IsScreenOnTop(this))
         {
-            _uiManager.PopScreen();
+            UIManager.PopScreen();
         }
     }
 }