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

Autoloads/UIManager.cs

@@ -4,7 +4,19 @@ namespace Mr.BrickAdventures.Autoloads;
 
 public partial class UIManager : Node
 {
+    public static UIManager Instance { get; private set; }
+
     private readonly System.Collections.Generic.List<Control> UiStack = new();
+
+    public override void _Ready()
+    {
+        Instance = this;
+    }
+
+    public override void _ExitTree()
+    {
+        if (Instance == this) Instance = null;
+    }
     
     [Signal] public delegate void ScreenPushedEventHandler(Control screen);
     [Signal] public delegate void ScreenPoppedEventHandler(Control screen);