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/controller/interfaces.go

@@ -96,7 +96,7 @@ type energyAdminUseCase interface {
 type commentUseCase interface {
 	CreateRoot(userID, postID uint, body string) (*model.Comment, error)
 	CreateReply(userID, parentID uint, body string) (*model.Comment, error)
-	Delete(commentID uint) error
+	Delete(commentID uint, requesterID uint, isAdmin bool) error
 	ListRootComments(postID uint, page, pageSize int) ([]model.Comment, int64, error)
 	ListReplies(rootID uint) ([]model.Comment, error)
 	SearchUsers(keyword string) ([]model.UserSearchResult, error)