Add achievement system with unlock and UI popup functionality (#8)

scripts/Resources/AchievementResource.cs

@@ -0,0 +1,13 @@
+using Godot;
+
+namespace Mr.BrickAdventures.scripts.Resources;
+
+[GlobalClass]
+public partial class AchievementResource : Resource
+{
+    [Export] public string Id { get; set; } = ""; // e.g., "level_1_complete"
+    [Export] public string DisplayName { get; set; } = "New Achievement";
+    [Export(PropertyHint.MultilineText)] public string Description { get; set; } = "";
+    [Export] public Texture2D Icon { get; set; }
+    [Export] public bool IsSecret { get; set; } = false;
+}