feat: 重构后台路由并使用嵌套路由；添加图片上传功能

- 拆分 AdminView 为多个子页面组件，使用嵌套路由
- 拆分 MerchantView 为多个子页面组件，使用嵌套路由
- 创建 AdminLayout 和 MerchantLayout 布局组件
- 修复编译错误：IconSend 重复导入、IconDatabase 不存在
- 修复 HomeView 缺失 onMounted 导入
- 添加文件上传后端接口和静态资源映射
- 为商品和轮播图添加图片上传功能（支持预览、清除）

frontend/src/api/index.js

@@ -61,5 +61,21 @@ export const api = {
   adminDeleteProduct: (id) => http.delete(`/api/admin/products/${id}`),
   adminReviews: () => http.get('/api/admin/reviews'),
   adminLogistics: () => http.get('/api/admin/logistics'),
-  adminInventory: () => http.get('/api/admin/inventory')
+  adminInventory: () => http.get('/api/admin/inventory'),
+
+  // File Upload
+  uploadImage: (file) => {
+    const formData = new FormData()
+    formData.append('file', file)
+    return http.post('/api/upload/image', formData, {
+      headers: { 'Content-Type': 'multipart/form-data' }
+    })
+  },
+  uploadFile: (file) => {
+    const formData = new FormData()
+    formData.append('file', file)
+    return http.post('/api/upload', formData, {
+      headers: { 'Content-Type': 'multipart/form-data' }
+    })
+  }
 }