refactor(image): 优化图片上传功能

- 修改后端接口，使用 @RequestPart 注解替代 @RequestParam
- 前端调整图片上传逻辑，直接发送 File 对象
- 简化前端上传成功后的处理流程

biji-houdaun/src/main/java/com/test/bijihoudaun/controller/ImageController.java

@@ -6,8 +6,6 @@ import com.test.bijihoudaun.common.response.R;
 import com.test.bijihoudaun.entity.Image;
 import com.test.bijihoudaun.service.ImageService;
 import io.swagger.v3.oas.annotations.Operation;
-import io.swagger.v3.oas.annotations.Parameter;
-import io.swagger.v3.oas.annotations.Parameters;
 import io.swagger.v3.oas.annotations.tags.Tag;
 import jakarta.servlet.http.HttpServletResponse;
 import org.springframework.beans.factory.annotation.Autowired;
@@ -35,7 +33,7 @@ public class ImageController {
     @Operation(summary = "上传图片")
     @PostMapping
     public R<Image> uploadImage(
-            @RequestParam("file") MultipartFile file,
+            @RequestPart("file") MultipartFile file,
             @RequestParam(value = "userId", required = false) Long userId,
             @RequestParam(value = "markdownId", required = false) Long markdownId) {