feat: Add quit callback support to engine and UI components

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

packages/wolf_3d_dart/lib/src/engine/wolf_3d_engine_base.dart

@@ -24,6 +24,7 @@ class WolfEngine {
     this.menuBackgroundRgb = 0x890000,
     this.menuPanelRgb = 0x590002,
     this.onMenuExit,
+    this.onQuit,
     this.onGameSelected,
     this.onEpisodeSelected,
     EngineAudio? engineAudio,
@@ -95,6 +96,9 @@ class WolfEngine {
   /// Callback triggered when backing out of the top-level menu.
   final void Function()? onMenuExit;
 
+  /// Callback triggered when the player explicitly selects QUIT.
+  final void Function()? onQuit;
+
   /// Callback triggered whenever the active game changes from menu flow.
   final void Function(WolfensteinData game)? onGameSelected;
 
@@ -330,8 +334,10 @@ class WolfEngine {
       case WolfMenuMainAction.backToGame:
         _resumeGame();
         break;
-      case WolfMenuMainAction.backToDemo:
       case WolfMenuMainAction.quit:
+        _quitProgram();
+        break;
+      case WolfMenuMainAction.backToDemo:
         _exitTopLevelMenu();
         break;
       case WolfMenuMainAction.sound:
@@ -445,6 +451,14 @@ class WolfEngine {
     onGameWon();
   }
 
+  void _quitProgram() {
+    if (onQuit != null) {
+      onQuit!.call();
+      return;
+    }
+    _exitTopLevelMenu();
+  }
+
   /// Wipes the current world state and builds a new floor from map data.
   void _loadLevel({required bool preservePlayerState}) {
     entities.clear();