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/service/comment_service.go

@@ -222,8 +222,9 @@ func (s *CommentService) parseMentions(commentID uint, body string) {
 		rows, _ := s.repo.SearchUsersByLevel(username, 2, 1)
 		if len(rows) > 0 {
 			_ = s.repo.CreateMention(&model.CommentMention{
-				CommentID: commentID,
-				UID:       rows[0].UID,
+				CommentID:        commentID,
+				UID:              rows[0].UID,
+				OriginalUsername: username,
 			})
 		}
 	}