feat: 前端配置代理解决上传跨域问题

- 配置 Vite 开发服务器代理 /api 和 /uploads 到后端
- API baseURL 改为相对路径 '/api'
- 修改上传 action 为相对路径 '/api/upload/image'
- 移除后端 FileUploadController 的 @CrossOrigin 注解
- 恢复 WebMvcConfig 的拦截器配置

frontend/src/views/admin/AdminProducts.vue

@@ -126,7 +126,7 @@ const merchantOptions = ref([])
 const productModalVisible = ref(false)
 const productForm = reactive({ id: null, name: '', category: '', description: '', price: 0, stock: 0, imageUrl: '', merchantId: null, approved: false })
 
-const uploadAction = 'http://localhost:8080/api/upload/image'
+const uploadAction = '/api/upload/image'
 const uploadHeaders = computed(() => ({
   'X-Token': localStorage.getItem('token') || ''
 }))