feat(flutter): 增加目录加载诊断与前台重试

test/discover/home_all_dramas_grid_test.dart

@@ -135,6 +135,9 @@ void main() {
 
     expect(find.byKey(const Key('home-categories-error')), findsOneWidget);
     expect(find.text('Retry'), findsOneWidget);
+    expect(find.byKey(const Key('catalog-debug-endpoint')), findsOneWidget);
+    expect(find.byKey(const Key('catalog-debug-error')), findsOneWidget);
+    expect(find.textContaining('discover feed unavailable'), findsOneWidget);
   });
 
   testWidgets('should_show_search_results_when_text_changes', (tester) async {