fix: 修复@提及uid始终为0的BUG + 空@提示优化

- SearchUsersByLevel SQL id列别名改为uid，修复GORM字段名映射导致uid=0
- @空输入时悬浮窗显示'输入以查找用户'而非空白
- 清理历史脏数据(uid=0的mention记录)

internal/repository/comment_mention_repo.go

@@ -80,7 +80,7 @@ func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int
 	threshold := model.LevelThresholds[minLevel]
 
 	err := r.db.Table("users").
-		Select("id, username, avatar, exp").
+		Select("id AS uid, username, avatar, exp").
 		Where("username LIKE ? AND exp >= ? AND deleted_at IS NULL", "%"+keyword+"%", threshold).
 		Order("exp DESC").
 		Limit(limit).