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）

backend/src/main/java/com/maternalmall/config/WebMvcConfig.java

@@ -1,25 +1,37 @@
 package com.maternalmall.config;
 
+import org.springframework.beans.factory.annotation.Value;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.web.servlet.config.annotation.CorsRegistry;
 import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
+import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
 
 @Configuration
 public class WebMvcConfig implements WebMvcConfigurer {
     private final AuthInterceptor authInterceptor;
 
+    @Value("${app.upload-path:./uploads}")
+    private String uploadPath;
+
     public WebMvcConfig(AuthInterceptor authInterceptor) {
         this.authInterceptor = authInterceptor;
     }
 
     @Override
     public void addInterceptors(InterceptorRegistry registry) {
-        registry.addInterceptor(authInterceptor).addPathPatterns("/**");
+        registry.addInterceptor(authInterceptor).addPathPatterns("/**")
+                .excludePathPatterns("/uploads/**");
     }
 
     @Override
     public void addCorsMappings(CorsRegistry registry) {
         registry.addMapping("/**").allowedOrigins("*").allowedMethods("*").allowedHeaders("*");
     }
+
+    @Override
+    public void addResourceHandlers(ResourceHandlerRegistry registry) {
+        registry.addResourceHandler("/uploads/**")
+                .addResourceLocations("file:" + uploadPath + "/");
+    }
 }