feat: @提及功能优化 + 默认头像改用SVG图标

- 评论区有效@提及渲染为蓝色链接跳转/space/{uid}，无效@保持纯文本
- 输入@触发悬浮窗，根据输入实时搜索用户（含头像+等级）
- 无匹配时显示未搜索到用户，选中后插入为@用户名 格式（尾部空格）
- 悬浮窗定位于输入光标上方，不遮挡输入内容
- 后端Comment新增Mentions字段，批量加载提及映射供前端判断
- SearchUsers/parseMentions 统一限制LV2+用户
- 所有默认头像由首字符文字改为统一SVG人形图标（nav/home/space/mention等7处）

internal/service/comment_service.go

@@ -152,7 +152,15 @@ func (s *CommentService) Delete(commentID uint, requesterID uint, isAdmin bool)
 func (s *CommentService) ListRootComments(postID uint, page, pageSize int) ([]model.Comment, int64, error) {
 	p := common.Pagination{Page: page, PageSize: pageSize}
 	p.DefaultPagination()
-	return s.repo.FindRootComments(postID, p.Offset(), p.PageSize)
+	comments, total, err := s.repo.FindRootComments(postID, p.Offset(), p.PageSize)
+	if err != nil {
+		return nil, 0, err
+	}
+	// 批量加载有效@提及映射，前端据此决定是否为链接
+	if len(comments) > 0 {
+		_ = s.repo.LoadMentionsForComments(comments)
+	}
+	return comments, total, nil
 }
 
 // ListReplies 获取某条顶级评论的全部回复
@@ -164,6 +172,10 @@ func (s *CommentService) ListReplies(rootID uint) ([]model.Comment, error) {
 	if list == nil {
 		list = []model.Comment{}
 	}
+	// 批量加载有效@提及映射
+	if len(list) > 0 {
+		_ = s.repo.LoadMentionsForComments(list)
+	}
 	return list, nil
 }
 
@@ -178,9 +190,9 @@ func (s *CommentService) ListAllComments(keyword string, showDeleted bool, page,
 	return rows, total, err
 }
 
-// SearchUsers 搜索LV3+用户（用于@艾特）
+// SearchUsers 搜索LV2+用户（用于@艾特悬浮窗）
 func (s *CommentService) SearchUsers(keyword string) ([]model.UserSearchResult, error) {
-	rows, err := s.repo.SearchUsersByLevel(keyword, 3, 10)
+	rows, err := s.repo.SearchUsersByLevel(keyword, 2, 10)
 	if err != nil {
 		return nil, fmt.Errorf("搜索用户失败: %w", err)
 	}
@@ -191,6 +203,7 @@ func (s *CommentService) SearchUsers(keyword string) ([]model.UserSearchResult,
 			UID:      row.UID,
 			Username: row.Username,
 			Level:    model.GetLevelByExp(row.Exp),
+			Avatar:   row.Avatar,
 		}
 	}
 	return users, nil
@@ -206,7 +219,7 @@ func (s *CommentService) parseMentions(commentID uint, body string) {
 			continue
 		}
 		seen[username] = true
-		rows, _ := s.repo.SearchUsersByLevel(username, 0, 1)
+		rows, _ := s.repo.SearchUsersByLevel(username, 2, 1)
 		if len(rows) > 0 {
 			_ = s.repo.CreateMention(&model.CommentMention{
 				CommentID: commentID,

internal/service/repository.go

@@ -158,9 +158,11 @@ type commentStore interface {
 	SearchUsersByLevel(keyword string, minLevel, limit int) ([]struct {
 		UID      uint
 		Username string
+		Avatar   string
 		Exp      int
 	}, error)
 	GetUsernameByID(userID uint) (string, error)
+	LoadMentionsForComments(comments []model.Comment) error
 	ListAllComments(keyword string, showDeleted bool, offset, limit int) ([]model.AdminCommentRow, int64, error)
 }