feat(service): 增加 Markdown 文件分组名称字段

- 在 MarkdownFileVO 类中添加 groupingName 字段
- 修改 getFilesByGroupingId 和 getRecentFiles 方法返回 MarkdownFileVO 列表- 在前端 HomePage 组件中显示分组名称
- 优化后端 SQL 查询，减少不必要的查询次数

biji-houdaun/src/main/java/com/test/bijihoudaun/mapper/MarkdownFileMapper.java

@@ -2,10 +2,27 @@ package com.test.bijihoudaun.mapper;
 
 import com.baomidou.mybatisplus.core.mapper.BaseMapper;
 import com.test.bijihoudaun.entity.MarkdownFile;
+import com.test.bijihoudaun.entity.MarkdownFileVO;
 import org.apache.ibatis.annotations.Mapper;
+import org.apache.ibatis.annotations.Param;
+import org.apache.ibatis.annotations.Select;
+
+import java.util.List;
 
 @Mapper
 public interface MarkdownFileMapper extends BaseMapper<MarkdownFile> {
-    // 自定义方法：根据用户ID获取文件列表
-    // List<MarkdownFile> findByUserId(Integer userId);
+
+    @Select("SELECT mf.*, g.grouping as groupingName " +
+            "FROM markdown_file mf " +
+            "LEFT JOIN grouping g ON mf.grouping_id = g.id " +
+            "ORDER BY mf.updated_at DESC " +
+            "LIMIT #{limit}")
+    List<MarkdownFileVO> selectRecentWithGrouping(@Param("limit") int limit);
+
+    @Select("SELECT mf.*, g.grouping as groupingName " +
+            "FROM markdown_file mf " +
+            "LEFT JOIN grouping g ON mf.grouping_id = g.id " +
+            "WHERE mf.grouping_id = #{groupingId} " +
+            "ORDER BY mf.updated_at DESC")
+    List<MarkdownFileVO> selectByGroupingIdWithGrouping(@Param("groupingId") String groupingId);
 }