Moved away from singleton pattern

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

apps/wolf_3d_gui/lib/screens/difficulty_screen.dart

@@ -4,8 +4,11 @@ import 'package:wolf_3d_flutter/wolf_3d.dart';
 import 'package:wolf_3d_gui/screens/game_screen.dart';
 
 class DifficultyScreen extends StatefulWidget {
+  final Wolf3d wolf3d;
+
   const DifficultyScreen({
     super.key,
+    required this.wolf3d,
   });
 
   @override
@@ -13,24 +16,25 @@ class DifficultyScreen extends StatefulWidget {
 }
 
 class _DifficultyScreenState extends State<DifficultyScreen> {
-  bool get isShareware => Wolf3d.I.activeGame.version == GameVersion.shareware;
+  bool get isShareware =>
+      widget.wolf3d.activeGame.version == GameVersion.shareware;
 
   @override
   void dispose() {
-    Wolf3d.I.audio.stopMusic();
+    widget.wolf3d.audio.stopMusic();
     super.dispose();
   }
 
   void _startGame(Difficulty difficulty, {bool showGallery = false}) {
-    Wolf3d.I.audio.stopMusic();
+    widget.wolf3d.audio.stopMusic();
 
     Navigator.of(context).pushReplacement(
       MaterialPageRoute(
         builder: (context) => GameScreen(
-          data: Wolf3d.I.activeGame,
+          data: widget.wolf3d.activeGame,
           difficulty: difficulty,
-          startingEpisode: Wolf3d.I.activeEpisode,
-          audio: Wolf3d.I.audio,
+          startingEpisode: widget.wolf3d.activeEpisode,
+          audio: widget.wolf3d.audio,
         ),
       ),
     );
@@ -59,8 +63,6 @@ class _DifficultyScreenState extends State<DifficultyScreen> {
               ),
             ),
             const SizedBox(height: 40),
-
-            // --- Difficulty Buttons ---
             ListView.builder(
               shrinkWrap: true,
               itemCount: Difficulty.values.length,