Add next level command and refactor player retrieval in GameManager; update scene files for consistency

Autoloads/GameManager.cs

@@ -11,9 +11,13 @@ public partial class GameManager : Node
 {
     [Export] public Array<PackedScene> LevelScenes { get; set; } = [];
     
-    public PlayerController Player { get; set; }
+    public PlayerController Player {
+        get => GetPlayer();
+        private set => _player = value;
+    }
     
     private List<Node> _sceneNodes = [];
+    private PlayerController _player;
     
     [Export] 
     public Dictionary PlayerState { get; set; } = new()
@@ -227,14 +231,14 @@ public partial class GameManager : Node
 
     public PlayerController GetPlayer()
     {
-        if (Player != null) return Player;
+        if (_player != null) return _player;
 
         foreach (var node in _sceneNodes)
         {
             if (node is not PlayerController player) continue;
             
-            Player = player;
-            return Player;
+            _player = player;
+            return _player;
         }
         
         GD.PrintErr("PlayerController not found in the scene tree.");