improve asnyc loading and error widgets

app/lib/features/settings/presentation/screens/web_engine_hardening.dart

@@ -129,7 +129,7 @@ class WebEngineHardeningScreen extends HookConsumerWidget {
             ),
           ),
         ),
-        loading: () => const SizedBox.shrink(),
+        loading: () => const Center(child: CircularProgressIndicator()),
       ),
     );
   }

app/lib/features/settings/presentation/screens/web_engine_hardening_group.dart

@@ -198,7 +198,7 @@ class WebEngineHardeningGroupScreen extends HookConsumerWidget {
             ),
           ),
         ),
-        loading: () => const SizedBox.shrink(),
+        loading: () => const Center(child: CircularProgressIndicator()),
       ),
     );
   }

app/lib/features/settings/presentation/widgets/default_search_selector.dart

@@ -83,7 +83,9 @@ class DefaultSearchSelector extends HookConsumerWidget {
           ),
         );
       },
-      error: (error, stackTrace) => const SizedBox.shrink(),
+      error: (error, stackTrace) => Center(
+        child: Icon(Icons.error, color: Theme.of(context).colorScheme.error),
+      ),
       loading: () => const SizedBox(height: 48, width: double.infinity),
     );
   }