fix: 评论系统全量检查修复（7 项 Gap 全部修复）

- Delete 权限：新增 requesterID/isAdmin 参数，Service 层鉴权（仅本人/管理员可删）
- 通知内容：改用 GetUsernameByID 显示真实用户名，非数字 UID
- SearchUsers：Repository 返回 exp，Service 调用 GetLevelByExp 计算等级
- 通知高亮：Notification 新增 CommentID 字段，消息链接支持 #comment-{id}
- 已删除提示：前端 hash 检测 + 3 秒超时 Toast "该评论已不存在"
- Admin CSS：移除不存在的 comments.css 引用
- 错误哨兵：改用 common.ErrCommentNotFound/ErrCommentEmpty/PermissionDenied

internal/repository/comment_repo.go

@@ -185,23 +185,23 @@ func (r *CommentRepo) FindMentionsByComment(commentID uint) ([]model.CommentMent
 	return list, err
 }
 
-// SearchUsersByLevel 搜索 LV3+ 用户（用于@艾特）
+// SearchUsersByLevel 搜索指定等级及以上用户（用于@艾特），返回 uid/username/exp
 func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int) ([]struct {
 	UID      string
 	Username string
+	Exp      int
 }, error) {
-	// LV3 = 1500 exp
 	type result struct {
-		UID      string
+		UID      uint
 		Username string
+		Exp      int
 	}
 	var list []result
 
-	// level 是计算字段，用 exp >= thresholds[minLevel] 判断
 	threshold := model.LevelThresholds[minLevel]
 
 	err := r.db.Table("users").
-		Select("uid, username").
+		Select("uid, username, exp").
 		Where("username LIKE ? AND exp >= ? AND deleted_at IS NULL", "%"+keyword+"%", threshold).
 		Order("exp DESC").
 		Limit(limit).
@@ -210,12 +210,14 @@ func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int
 	var res []struct {
 		UID      string
 		Username string
+		Exp      int
 	}
 	for _, u := range list {
 		res = append(res, struct {
 			UID      string
 			Username string
-		}{UID: u.UID, Username: u.Username})
+			Exp      int
+		}{UID: strconv.FormatUint(uint64(u.UID), 10), Username: u.Username, Exp: u.Exp})
 	}
 	return res, err
 }
@@ -230,6 +232,13 @@ func (r *CommentRepo) GetUserUIDByID(userID uint) (string, error) {
 	return strconv.FormatUint(uint64(uid), 10), nil
 }
 
+// GetUsernameByID 通过 userID 获取用户名（用于通知内容）
+func (r *CommentRepo) GetUsernameByID(userID uint) (string, error) {
+	var username string
+	err := r.db.Table("users").Select("username").Where("uid = ?", userID).Scan(&username).Error
+	return username, err
+}
+
 // FindPostIDByComment 查询评论所属的文章ID
 func (r *CommentRepo) FindPostIDByComment(commentID uint) (uint, error) {
 	var postID uint