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/repository/comment_mention_repo.go

@@ -17,15 +17,62 @@ func (r *CommentRepo) FindMentionsByComment(commentID uint) ([]model.CommentMent
 	return list, err
 }
 
-// SearchUsersByLevel 搜索指定等级及以上用户（用于@艾特），返回 uid/username/exp
+// LoadMentionsForComments 批量加载多条评论的有效@提及映射（username -> uid）
+// 传入的 comments 会被原地修改，填充 Mentions 字段
+func (r *CommentRepo) LoadMentionsForComments(comments []model.Comment) error {
+	if len(comments) == 0 {
+		return nil
+	}
+
+	commentIDs := make([]uint, len(comments))
+	for i, c := range comments {
+		commentIDs[i] = c.ID
+	}
+
+	// 查询 comment_mentions JOIN users，拿到 username 和 uid
+	type mentionRow struct {
+		CommentID uint
+		UID       uint
+		Username  string
+	}
+	var rows []mentionRow
+	err := r.db.Table("comment_mentions").
+		Select("comment_mentions.comment_id, comment_mentions.uid, users.username").
+		Joins("LEFT JOIN users ON users.id = comment_mentions.uid").
+		Where("comment_mentions.comment_id IN ?", commentIDs).
+		Find(&rows).Error
+	if err != nil {
+		return err
+	}
+
+	// 按 comment_id 分组
+	mentionMap := make(map[uint]map[string]uint)
+	for _, row := range rows {
+		if mentionMap[row.CommentID] == nil {
+			mentionMap[row.CommentID] = make(map[string]uint)
+		}
+		mentionMap[row.CommentID][row.Username] = row.UID
+	}
+
+	for i := range comments {
+		if m, ok := mentionMap[comments[i].ID]; ok {
+			comments[i].Mentions = m
+		}
+	}
+	return nil
+}
+
+// SearchUsersByLevel 搜索指定等级及以上用户（用于@艾特），返回 uid/username/avatar/exp
 func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int) ([]struct {
 	UID      uint
 	Username string
+	Avatar   string
 	Exp      int
 }, error) {
 	type result struct {
 		UID      uint
 		Username string
+		Avatar   string
 		Exp      int
 	}
 	var list []result
@@ -33,7 +80,7 @@ func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int
 	threshold := model.LevelThresholds[minLevel]
 
 	err := r.db.Table("users").
-		Select("id, username, exp").
+		Select("id, username, avatar, exp").
 		Where("username LIKE ? AND exp >= ? AND deleted_at IS NULL", "%"+keyword+"%", threshold).
 		Order("exp DESC").
 		Limit(limit).
@@ -42,14 +89,16 @@ func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int
 	var res []struct {
 		UID      uint
 		Username string
+		Avatar   string
 		Exp      int
 	}
 	for _, u := range list {
 		res = append(res, struct {
 			UID      uint
 			Username string
+			Avatar   string
 			Exp      int
-		}{UID: u.UID, Username: u.Username, Exp: u.Exp})
+		}{UID: u.UID, Username: u.Username, Avatar: u.Avatar, Exp: u.Exp})
 	}
 	return res, err
 }