Move level count to getter

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

lib/features/screens/episode_screen.dart

@@ -36,10 +36,7 @@ class _EpisodeScreenState extends State<EpisodeScreen> {
 
   @override
   Widget build(BuildContext context) {
-    final int numberOfEpisodes = (Wolf3d.I.levels.length / 10).floor().clamp(
-      1,
-      6,
-    );
+    final int numberOfEpisodes = Wolf3d.I.activeGame.numberOfEpisodes;
 
     return Scaffold(
       backgroundColor: Colors.black,

packages/wolf_3d_data_types/lib/src/wolfenstein_data.dart

@@ -10,6 +10,11 @@ class WolfensteinData {
   final List<WolfLevel> levels;
   final List<VgaImage> vgaImages;
 
+  // --- Derived Properties ---
+  /// Calculates the number of available episodes based on the loaded levels.
+  /// (Each episode consists of exactly 10 levels).
+  int get numberOfEpisodes => (levels.length / 10).floor().clamp(1, 6);
+
   const WolfensteinData({
     required this.version,
     required this.walls,