feat: 添加每日签到功能，新增帖子排序与社区面板优化

本次提交完成了多个核心功能迭代：
1.  新增用户每日签到系统，包含签到状态查询、连续天数统计、积分累计逻辑
2.  新增"新帖子"排序选项，替换原有的latest排序为新评论排序
3.  重构首页信息流布局，优化移动端与桌面端展示样式
4.  新增最新评论、最新注册用户模块，完善社区面板数据展示
5.  新增静态友情链接配置模块
6.  提取通用时间格式化工具函数，统一全站相对时间展示
7.  补充数据库签到模型与后端API接口，完善前后端交互链路

backend/service/overview.go

@@ -1,6 +1,7 @@
 package service
 
 import (
+	"strings"
 	"time"
 
 	"github.com/freefire/jiang13-bbs/model"
@@ -39,12 +40,51 @@ type BoardCount struct {
 	PostCount int64 `json:"post_count"`
 }
 
+// LatestCommentItem 首页"最新评论"条目（纯文本摘要 + 所属帖子）
+type LatestCommentItem struct {
+	ID        uint      `json:"id"`
+	Content   string    `json:"content"`
+	CreatedAt time.Time `json:"created_at"`
+	User      struct {
+		ID       uint   `json:"id"`
+		Username string `json:"username"`
+		Nickname string `json:"nickname"`
+		Avatar   string `json:"avatar"`
+	} `json:"user"`
+	Post struct {
+		ID    uint   `json:"id"`
+		Title string `json:"title"`
+	} `json:"post"`
+}
+
+// NewUserItem 最新注册成员
+type NewUserItem struct {
+	ID        uint      `json:"id"`
+	Username  string    `json:"username"`
+	Nickname  string    `json:"nickname"`
+	Avatar    string    `json:"avatar"`
+	CreatedAt time.Time `json:"created_at"`
+}
+
 // OverviewData 首页聚合数据
 type OverviewData struct {
-	Stats       OverviewStats  `json:"stats"`
-	Hot         []PostListItem `json:"hot"`
-	ActiveUsers []ActiveUser   `json:"active_users"`
-	Boards      []BoardCount   `json:"boards"`
+	Stats          OverviewStats       `json:"stats"`
+	Hot            []PostListItem      `json:"hot"`
+	ActiveUsers    []ActiveUser        `json:"active_users"`
+	Boards         []BoardCount        `json:"boards"`
+	LatestComments []LatestCommentItem `json:"latest_comments"`
+	NewUsers       []NewUserItem       `json:"new_users"`
+	Checkin        *CheckinStatus      `json:"checkin,omitempty"`
+}
+
+// truncateText 折叠空白并按 rune 截断（评论为纯文本，无 HTML 注入面）
+func truncateText(s string, n int) string {
+	s = strings.Join(strings.Fields(s), " ")
+	r := []rune(s)
+	if len(r) > n {
+		return string(r[:n]) + "…"
+	}
+	return s
 }
 
 const activeUserWindow = 14 * 24 * time.Hour
@@ -52,9 +92,11 @@ const activeUserWindow = 14 * 24 * time.Hour
 // Get 聚合首页数据。hot 的 liked 态由 handler 层按当前用户填充。
 func (s *OverviewService) Get() (*OverviewData, error) {
 	data := &OverviewData{
-		Hot:         []PostListItem{},
-		ActiveUsers: []ActiveUser{},
-		Boards:      []BoardCount{},
+		Hot:            []PostListItem{},
+		ActiveUsers:    []ActiveUser{},
+		Boards:         []BoardCount{},
+		LatestComments: []LatestCommentItem{},
+		NewUsers:       []NewUserItem{},
 	}
 
 	now := time.Now()
@@ -163,5 +205,65 @@ LIMIT 5`
 		data.Boards = append(data.Boards, BoardCount{Board: b, PostCount: countMap[b.ID]})
 	}
 
+	// 最新评论：JOIN 未封禁用户与已发布帖，一条 SQL 取齐
+	type latestCommentRow struct {
+		ID        uint
+		Content   string
+		CreatedAt time.Time
+		UID       uint
+		Username  string
+		Nickname  string
+		Avatar    string
+		PID       uint
+		Title     string
+	}
+	var commentRows []latestCommentRow
+	commentSQL := `
+SELECT c.id, c.content, c.created_at,
+       u.id AS uid, u.username AS username, u.nickname AS nickname, u.avatar AS avatar,
+       p.id AS pid, p.title AS title
+FROM comments c
+JOIN users u ON u.id = c.user_id AND u.banned = ? AND u.deleted_at IS NULL
+JOIN posts p ON p.id = c.post_id AND p.status = ? AND p.deleted_at IS NULL
+WHERE c.status = ? AND c.deleted_at IS NULL
+ORDER BY c.created_at DESC
+LIMIT 6`
+	if err := s.db.Raw(commentSQL,
+		false, model.ContentStatusPublished, model.ContentStatusPublished).
+		Scan(&commentRows).Error; err != nil {
+		return nil, err
+	}
+	for _, r := range commentRows {
+		item := LatestCommentItem{
+			ID:        r.ID,
+			Content:   truncateText(r.Content, 60),
+			CreatedAt: r.CreatedAt,
+		}
+		item.User.ID = r.UID
+		item.User.Username = r.Username
+		item.User.Nickname = r.Nickname
+		item.User.Avatar = r.Avatar
+		item.Post.ID = r.PID
+		item.Post.Title = r.Title
+		data.LatestComments = append(data.LatestComments, item)
+	}
+
+	// 最新注册成员
+	var newUsers []model.User
+	if err := s.db.Select("id, username, nickname, avatar, created_at").
+		Where("banned = ?", false).
+		Order("created_at DESC").Limit(8).Find(&newUsers).Error; err != nil {
+		return nil, err
+	}
+	for _, u := range newUsers {
+		data.NewUsers = append(data.NewUsers, NewUserItem{
+			ID:        u.ID,
+			Username:  u.Username,
+			Nickname:  u.Nickname,
+			Avatar:    u.Avatar,
+			CreatedAt: u.CreatedAt,
+		})
+	}
+
 	return data, nil
 }