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"

backend/data/system-sites.json

@@ -0,0 +1,62 @@
+[
+  {
+    "id": "github",
+    "title": "GitHub",
+    "url": "https://github.com",
+    "description": "代码托管与开源社区",
+    "categoryId": "common",
+    "source": "system",
+    "order": 10,
+    "createdAt": "2026-09-13T10:29:48Z",
+    "updatedAt": "2026-09-13T10:29:48Z",
+    "submittedBy": ""
+  },
+  {
+    "id": "zhihu",
+    "title": "知乎",
+    "url": "https://www.zhihu.com",
+    "description": "深度内容问答社区",
+    "categoryId": "common",
+    "source": "system",
+    "order": 20,
+    "createdAt": "2026-09-13T10:29:48Z",
+    "updatedAt": "2026-09-13T10:29:48Z",
+    "submittedBy": ""
+  },
+  {
+    "id": "bilibili",
+    "title": "哔哩哔哩",
+    "url": "https://www.bilibili.com",
+    "description": "弹幕视频分享网站",
+    "categoryId": "common",
+    "source": "system",
+    "order": 30,
+    "createdAt": "2026-09-13T10:29:48Z",
+    "updatedAt": "2026-09-13T10:29:48Z",
+    "submittedBy": ""
+  },
+  {
+    "id": "figma",
+    "title": "Figma",
+    "url": "https://www.figma.com",
+    "description": "协同界面设计工具",
+    "categoryId": "design",
+    "source": "system",
+    "order": 10,
+    "createdAt": "2026-09-13T10:29:48Z",
+    "updatedAt": "2026-09-13T10:29:48Z",
+    "submittedBy": ""
+  },
+  {
+    "id": "dribbble",
+    "title": "Dribbble",
+    "url": "https://dribbble.com",
+    "description": "全球设计作品分享社区",
+    "categoryId": "design",
+    "source": "system",
+    "order": 20,
+    "createdAt": "2026-09-13T10:29:48Z",
+    "updatedAt": "2026-09-13T10:29:48Z",
+    "submittedBy": ""
+  }
+]