fix: preview player lifecycle and minor safety cleanups

- search_screen caches PreviewPlayerController in initState to avoid
  ref access during dispose
- extension_provider: convert enabledExtensions/searchProviders getters
  to methods
- artist_screen: drop redundant null assertions on motion banner inputs

lib/providers/extension_provider.dart

@@ -1670,7 +1670,7 @@ class ExtensionNotifier extends Notifier<ExtensionState> {
     }
   }
 
-  List<Extension> get enabledExtensions {
+  List<Extension> enabledExtensions() {
     return state.extensions.where((ext) => ext.enabled).toList();
   }
 
@@ -1747,7 +1747,7 @@ class ExtensionNotifier extends Notifier<ExtensionState> {
     return result;
   }
 
-  List<Extension> get searchProviders {
+  List<Extension> searchProviders() {
     return state.extensions
         .where((ext) => ext.enabled && ext.hasCustomSearch)
         .toList();
@@ -1761,9 +1761,7 @@ class ExtensionNotifier extends Notifier<ExtensionState> {
     void scan(Object? settingsList) {
       if (settingsList is! List) return;
       for (final entry in settingsList) {
-        if (entry is Map &&
-            entry['secret'] == true &&
-            entry['key'] is String) {
+        if (entry is Map && entry['secret'] == true && entry['key'] is String) {
           keys.add(entry['key'] as String);
         }
       }

lib/screens/artist_screen.dart

@@ -1206,10 +1206,10 @@ class _ArtistScreenState extends ConsumerState<ArtistScreen> {
           children: [
             if (hasMotionBanner)
               MotionHeaderBanner(
-                videoUrl: headerVideoUrl!,
+                videoUrl: headerVideoUrl,
                 fallback: hasValidImage
                     ? CachedCoverImage(
-                        imageUrl: imageUrl!,
+                        imageUrl: imageUrl,
                         fit: BoxFit.cover,
                         alignment: Alignment.topCenter,
                         memCacheWidth: 800,

lib/screens/search_screen.dart

@@ -25,11 +25,13 @@ class SearchScreen extends ConsumerStatefulWidget {
 
 class _SearchScreenState extends ConsumerState<SearchScreen> {
   late TextEditingController _searchController;
+  late PreviewPlayerController _previewPlayerController;
 
   @override
   void initState() {
     super.initState();
     _searchController = TextEditingController(text: widget.query);
+    _previewPlayerController = ref.read(previewPlayerProvider.notifier);
     if (widget.query.isNotEmpty) {
       WidgetsBinding.instance.addPostFrameCallback((_) {
         ref.read(trackProvider.notifier).search(widget.query);
@@ -39,7 +41,7 @@ class _SearchScreenState extends ConsumerState<SearchScreen> {
 
   @override
   void dispose() {
-    ref.read(previewPlayerProvider.notifier).stop();
+    _previewPlayerController.stop();
     _searchController.dispose();
     super.dispose();
   }