feat: Add quit callback support to engine and UI components

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

packages/wolf_3d_flutter/lib/wolf_3d_flutter.dart

@@ -82,7 +82,10 @@ class Wolf3d {
   /// Uses [activeGame], [activeEpisode], and [activeDifficulty]. Stores the
   /// engine so it can be retrieved via [engine]. [onGameWon] is invoked when
   /// the player completes the final level of the episode.
-  WolfEngine launchEngine({required void Function() onGameWon}) {
+  WolfEngine launchEngine({
+    required void Function() onGameWon,
+    void Function()? onQuit,
+  }) {
     if (availableGames.isEmpty) {
       throw StateError(
         'No game data was discovered. Add game files before launching the engine.',
@@ -102,6 +105,7 @@ class Wolf3d {
       // In Flutter we keep the renderer screen active while browsing menus,
       // so backing out of the top-level menu should not pop the route.
       onMenuExit: () {},
+      onQuit: onQuit,
       onGameSelected: (game) {
         _activeGame = game;
         audio.activeGame = game;