feat: @提及支持改名后链接跟随UID + 显示名自动更新

- comment_mentions新增original_username列，创建时记录原始@用户名
- Mentions map改为{original_username: {uid, name}}结构，name为当前显示名
- 用户改名后：旧@文本仍能匹配original_username，链接跟随uid不变，显示名自动更新
- 前端renderMentions适配新结构

internal/repository/comment_mention_repo.go

@@ -17,8 +17,9 @@ func (r *CommentRepo) FindMentionsByComment(commentID uint) ([]model.CommentMent
 	return list, err
 }
 
-// LoadMentionsForComments 批量加载多条评论的有效@提及映射（username -> uid）
-// 传入的 comments 会被原地修改，填充 Mentions 字段
+// LoadMentionsForComments 批量加载多条评论的有效@提及映射
+// key=创建时的原始@用户名, value={uid, 用户当前显示名}
+// 用户改名后：原始名匹配body中的旧@文本，显示名随users表更新
 func (r *CommentRepo) LoadMentionsForComments(comments []model.Comment) error {
 	if len(comments) == 0 {
 		return nil
@@ -29,15 +30,15 @@ func (r *CommentRepo) LoadMentionsForComments(comments []model.Comment) error {
 		commentIDs[i] = c.ID
 	}
 
-	// 查询 comment_mentions JOIN users，拿到 username 和 uid
 	type mentionRow struct {
-		CommentID uint
-		UID       uint
-		Username  string
+		CommentID        uint
+		UID              uint
+		OriginalUsername string
+		CurrentName      string
 	}
 	var rows []mentionRow
 	err := r.db.Table("comment_mentions").
-		Select("comment_mentions.comment_id, comment_mentions.uid, users.username").
+		Select("comment_mentions.comment_id, comment_mentions.uid, comment_mentions.original_username, users.username AS current_name").
 		Joins("LEFT JOIN users ON users.id = comment_mentions.uid").
 		Where("comment_mentions.comment_id IN ?", commentIDs).
 		Find(&rows).Error
@@ -45,13 +46,16 @@ func (r *CommentRepo) LoadMentionsForComments(comments []model.Comment) error {
 		return err
 	}
 
-	// 按 comment_id 分组
-	mentionMap := make(map[uint]map[string]uint)
+	// 按 comment_id 分组，key = original_username, value = MentionInfo
+	mentionMap := make(map[uint]map[string]model.MentionInfo)
 	for _, row := range rows {
 		if mentionMap[row.CommentID] == nil {
-			mentionMap[row.CommentID] = make(map[string]uint)
+			mentionMap[row.CommentID] = make(map[string]model.MentionInfo)
+		}
+		mentionMap[row.CommentID][row.OriginalUsername] = model.MentionInfo{
+			UID:  row.UID,
+			Name: row.CurrentName,
 		}
-		mentionMap[row.CommentID][row.Username] = row.UID
 	}
 
 	for i := range comments {