fix: 添加动态分类页面路由和模板

修复问题:
- 新增类别的详情页面路由不存在
- 导航栏缺少后台入口

新增:
- /category/{id} 路由
- category.html 模板页面
- 导航栏添加后台入口

data/categories.json

@@ -38,5 +38,14 @@
     "color": "teal",
     "description": "笔记本电脑、台式机参数",
     "order": 5
+  },
+  {
+    "id": "021dc76d36be",
+    "name": "汽车",
+    "icon": "ri-folder-line",
+    "color": "blue",
+    "order": 6,
+    "description": "汽车方面",
+    "created_at": "2026-04-09 10:09:01"
   }
 ]

data/items_021dc76d36be.json

@@ -0,0 +1,11 @@
+[
+  {
+    "name": "比亚迪宋plus dmi 2021款",
+    "brand": "比亚迪",
+    "price": "18.87",
+    "year": "2021",
+    "category_id": "021dc76d36be",
+    "id": "3d20dbcd4bdd",
+    "created_at": "2026-04-09 10:09:56"
+  }
+]