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/service/repository.go

@@ -86,7 +86,7 @@ type taskStore interface {
 
 // levelNotifier 等级升级通知接口
 type levelNotifier interface {
-	Create(userID uint, notifyType, title, content string, relatedID *uint) error
+	Create(userID uint, notifyType, title, content string, relatedID, commentID *uint) error
 }
 
 // EnergyStore EnergyService 所需的完整仓储接口
@@ -113,7 +113,7 @@ type energyStore = EnergyStore
 
 // energyNotifier EnergyService 所需的通知接口（ISP）
 type energyNotifier interface {
-	Create(userID uint, notifyType, title, content string, relatedID *uint) error
+	Create(userID uint, notifyType, title, content string, relatedID, commentID *uint) error
 }
 
 // commentStore CommentService 所需的最小仓储接口（ISP）
@@ -132,12 +132,14 @@ type commentStore interface {
 	SearchUsersByLevel(keyword string, minLevel, limit int) ([]struct {
 		UID      string
 		Username string
+		Exp      int
 	}, error)
 	GetUserUIDByID(userID uint) (string, error)
+	GetUsernameByID(userID uint) (string, error)
 	ListAllComments(keyword string, showDeleted bool, offset, limit int) ([]model.AdminCommentRow, int64, error)
 }
 
 // commentNotifier CommentService 所需的通知接口（ISP）
 type commentNotifier interface {
-	Create(userID uint, notifyType, title, content string, relatedID *uint) error
+	Create(userID uint, notifyType, title, content string, relatedID, commentID *uint) error
 }