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

Autoloads/AchievementManager.cs

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+using Godot;
+using Godot.Collections;
+using Mr.BrickAdventures.scripts.Resources;
+
+namespace Mr.BrickAdventures.Autoloads;
+
+public partial class AchievementManager : Node
+{
+    [Export] private string AchievementsFolderPath = "res://achievements/";
+    [Export] private PackedScene AchievementPopupScene { get; set; }
+    
+    private System.Collections.Generic.Dictionary<string, AchievementResource> _achievements = new();
+    private Array<string> _unlockedAchievementIds = [];
+    private GameManager _gameManager;
+    
+    public override void _Ready()
+    {
+        _gameManager = GetNode<GameManager>("/root/GameManager");
+        LoadAchievementsFromFolder();
+        LoadUnlockedAchievements();
+    }
+
+    private void LoadAchievementsFromFolder()
+    {
+        using var dir = DirAccess.Open(AchievementsFolderPath);
+        if (dir == null)
+        {
+            GD.PrintErr($"AchievementManager: Could not open achievements folder at '{AchievementsFolderPath}'");
+            return;
+        }
+
+        dir.ListDirBegin();
+        var fileName = dir.GetNext();
+        while (fileName != "")
+        {
+            if (!dir.CurrentIsDir() && fileName.EndsWith(".tres"))
+            {
+                var achievement = GD.Load<AchievementResource>(AchievementsFolderPath + fileName);
+                if (achievement != null)
+                {
+                    _achievements.TryAdd(achievement.Id, achievement);
+                }
+            }
+            fileName = dir.GetNext();
+        }
+    }
+    
+    public void UnlockAchievement(string achievementId)
+    {
+        if (!_achievements.TryGetValue(achievementId, out var achievement))
+        {
+            GD.PrintErr($"Attempted to unlock non-existent achievement: '{achievementId}'");
+            return;
+        }
+
+        if (_unlockedAchievementIds.Contains(achievementId))
+        {
+            return; // Already unlocked
+        }
+
+        // 1. Mark as unlocked internally
+        _unlockedAchievementIds.Add(achievementId);
+        GD.Print($"Achievement Unlocked: {achievement.DisplayName}");
+
+        // 2. Show the UI popup
+        if (AchievementPopupScene != null)
+        {
+            var popup = AchievementPopupScene.Instantiate<scripts.UI.AchievementPopup>();
+            GetTree().Root.AddChild(popup);
+            _ = popup.ShowAchievement(achievement);
+        }
+
+        // 3. Call SteamManager if it's available
+        if (SteamManager.IsSteamInitialized)
+        {
+            SteamManager.UnlockAchievement(achievement.Id);
+        }
+        
+        // 4. Save progress
+        SaveUnlockedAchievements();
+    }
+    
+    public void LockAchievement(string achievementId)
+    {
+        if (_unlockedAchievementIds.Contains(achievementId))
+        {
+            _unlockedAchievementIds.Remove(achievementId);
+            SaveUnlockedAchievements();
+        }
+    }
+    
+    private void SaveUnlockedAchievements()
+    {
+        _gameManager.PlayerState["unlocked_achievements"] = _unlockedAchievementIds;
+        // You might want to trigger a save game here, depending on your SaveSystem
+    }
+
+    private void LoadUnlockedAchievements()
+    {
+        if (_gameManager.PlayerState.TryGetValue("unlocked_achievements", out var unlocked))
+        {
+            _unlockedAchievementIds = (Array<string>)unlocked;
+        }
+    }
+}