refactor(empty-state): 空状态支持内容居中

test/library/favorites_page_test.dart

@@ -174,6 +174,14 @@ void main() {
 
     expect(find.byKey(const Key('favorites-empty')), findsOneWidget);
     expect(find.byType(EmptyState), findsOneWidget);
+    final viewport = tester.getRect(
+      find.byKey(const Key('favorites-empty-scroll')),
+    );
+    final content = tester.getRect(
+      find.byKey(const Key('empty-state-content')),
+    );
+    expect(content.center.dx, closeTo(viewport.center.dx, 0.1));
+    expect(content.center.dy, closeTo(viewport.center.dy, 0.1));
   });
 
   testWidgets('should_show_retry_when_favorite_list_fails', (tester) async {