feat: 实现导航页基础功能

- 添加导航条目和分类数据模型
- 实现导航卡片网格展示
- 支持点击卡片打开链接
- 添加主题样式（渐变色、卡片阴影）
- 区分系统默认和自定义条目
- 添加分类统计信息"

frontend/lib/features/home/data/http_home_repository.dart

@@ -149,6 +149,7 @@ HomeData _toHomeData(List<Object?> groups, Map<String, dynamic> siteMeta) {
     searchHint: siteMeta['searchHint'] as String? ?? _fallbackSearchHint,
     navNodes: navNodes,
     sections: sections,
+    submitUrl: siteMeta['submitUrl'] as String? ?? '',
   );
 }
 

frontend/lib/features/home/data/mock_home_data.dart

@@ -80,4 +80,5 @@ const HomeData mockHomeData = HomeData(
   searchHint: '输入关键字搜索全网资源...',
   navNodes: mockNavNodes,
   sections: mockLinkSections,
+  submitUrl: 'https://mattermost.example.com',
 );