improve asnyc loading and error widgets

app/lib/features/web_feed/presentation/screens/feed_article_list.dart

@@ -190,7 +190,7 @@ class FeedArticleListScreen extends HookConsumerWidget {
               exception: error,
             ),
           ),
-          loading: () => const SizedBox.shrink(),
+          loading: () => const Center(child: CircularProgressIndicator()),
         ),
       ),
     );

app/lib/features/web_feed/presentation/screens/feed_list.dart

@@ -93,7 +93,7 @@ class FeedListScreen extends HookConsumerWidget {
             },
           ),
         ),
-        loading: () => const SizedBox.shrink(),
+        loading: () => const Center(child: CircularProgressIndicator()),
       ),
       floatingActionButton: FloatingActionButton.extended(
         label: const Text('Feed'),