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/controller/FileUploadController.java

@@ -0,0 +1,73 @@
+package com.maternalmall.controller;
+
+import com.maternalmall.common.ApiResponse;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.web.bind.annotation.*;
+import org.springframework.web.multipart.MultipartFile;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.UUID;
+
+@RestController
+@RequestMapping("/api")
+public class FileUploadController {
+
+    @Value("${app.upload-path:./uploads}")
+    private String uploadPath;
+
+    @Value("${app.upload-url-prefix:/uploads}")
+    private String uploadUrlPrefix;
+
+    @PostMapping("/upload")
+    public ApiResponse<String> uploadFile(@RequestParam("file") MultipartFile file) {
+        if (file.isEmpty()) {
+            return ApiResponse.error("请选择要上传的文件");
+        }
+
+        try {
+            // 创建上传目录
+            Path uploadDir = Paths.get(uploadPath);
+            if (!Files.exists(uploadDir)) {
+                Files.createDirectories(uploadDir);
+            }
+
+            // 生成唯一文件名
+            String originalFilename = file.getOriginalFilename();
+            String extension = "";
+            if (originalFilename != null && originalFilename.contains(".")) {
+                extension = originalFilename.substring(originalFilename.lastIndexOf("."));
+            }
+            String newFilename = UUID.randomUUID().toString().replace("-", "") + extension;
+
+            // 保存文件
+            Path filePath = uploadDir.resolve(newFilename);
+            file.transferTo(filePath.toFile());
+
+            // 返回文件URL
+            String fileUrl = uploadUrlPrefix + "/" + newFilename;
+            return ApiResponse.success(fileUrl);
+
+        } catch (IOException e) {
+            return ApiResponse.error("文件上传失败: " + e.getMessage());
+        }
+    }
+
+    @PostMapping("/upload/image")
+    public ApiResponse<String> uploadImage(@RequestParam("file") MultipartFile file) {
+        if (file.isEmpty()) {
+            return ApiResponse.error("请选择要上传的图片");
+        }
+
+        // 检查文件类型
+        String contentType = file.getContentType();
+        if (contentType == null || !contentType.startsWith("image/")) {
+            return ApiResponse.error("只能上传图片文件");
+        }
+
+        return uploadFile(file);
+    }
+}