refactor: fix bugs and improve architecture

- Fix double-execution bug in LevelStateHandler (coins/skills were committed twice per level)
- Fix DamageComponent to track multiple targets via HashSet instead of single node
- Fix HealthComponent: update health immediately, decouple from PlayerController via signal wiring in PlayerController
- Remove dead loop in SkillManager.RemoveSkill; simplify O(n²) throw skill loop
- Replace GetNode<T>(path) with .Instance across managers; add Instance to StatisticsManager/SpeedRunManager
- GameManager.GetPlayer() now uses EventBus.PlayerSpawned instead of scanning all scene nodes
- UIManager.UiStack: remove [Export], use private List<Control>
- PlayerState: extract DefaultLives constant, simplify CreateDefault()

scripts/Events/StatisticsEventHandler.cs

@@ -1,5 +1,4 @@
 using Godot;
-using Mr.BrickAdventures;
 using Mr.BrickAdventures.Autoloads;
 
 namespace Mr.BrickAdventures.scripts.Events;
@@ -14,7 +13,7 @@ public partial class StatisticsEventHandler : Node
 
     public override void _Ready()
     {
-        _statisticsManager = GetNode<StatisticsManager>(Constants.StatisticsManagerPath);
+        _statisticsManager = StatisticsManager.Instance;
 
         // Subscribe to events
         EventBus.Instance.CoinCollected += OnCoinCollected;