外观设置改为半屏布局；背景图仅保存后生效，并支持媒体库选用。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/service/upload.go

@@ -364,6 +364,24 @@ func (s *UploadService) SaveBackground(src io.Reader) (string, error) {
 	return "/uploads/backgrounds/" + filename, nil
 }
 
+// CopyBackgroundFromMedia 把当前用户媒体库里的一张图复制进 backgrounds，不写站点设置
+func (s *UploadService) CopyBackgroundFromMedia(userID, attachmentID uint) (string, error) {
+	var att model.Attachment
+	if err := s.db.Where("id = ? AND user_id = ?", attachmentID, userID).First(&att).Error; err != nil {
+		return "", errors.New("图片不存在或不属于你")
+	}
+	abs, ok := s.safeUploadPath(att.URL)
+	if !ok {
+		return "", errors.New("无效的图片地址")
+	}
+	f, err := os.Open(abs)
+	if err != nil {
+		return "", errors.New("读取图片失败")
+	}
+	defer f.Close()
+	return s.SaveBackground(f)
+}
+
 // BackgroundFileExists 确认 URL 对应文件在 backgrounds 目录内
 func (s *UploadService) BackgroundFileExists(url string) bool {
 	rel := BgRelPath(url)