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

frontend/lib/api.ts

@@ -63,11 +63,37 @@ export interface BoardWithCount extends Board {
   post_count: number;
 }
 
+export interface LatestComment {
+  id: number;
+  content: string;
+  created_at: string;
+  user: { id: number; username: string; nickname: string; avatar: string };
+  post: { id: number; title: string };
+}
+
+export interface NewUser {
+  id: number;
+  username: string;
+  nickname: string;
+  avatar: string;
+  created_at: string;
+}
+
+export interface CheckinStatus {
+  checked_today: boolean;
+  streak: number;
+  total_points: number;
+  today_points: number;
+}
+
 export interface OverviewResponse {
   stats: OverviewStats;
   hot: PostListItem[];
   active_users: ActiveUser[];
   boards: BoardWithCount[];
+  latest_comments: LatestComment[];
+  new_users: NewUser[];
+  checkin?: CheckinStatus | null;
 }
 
 export interface PostListItem {
@@ -475,3 +501,24 @@ export async function apiMarkAllRead() {
   });
   return res.json();
 }
+
+// ===== 每日签到 =====
+
+export async function apiGetCheckin(): Promise<CheckinStatus> {
+  const res = await fetchWithRefresh("/api/checkin", {
+    headers: clientHeaders(),
+  });
+  if (!res.ok) throw new Error("获取签到状态失败");
+  return res.json();
+}
+
+// 执行签到；失败（如今日已签，409）抛出后端消息
+export async function apiDoCheckin(): Promise<CheckinStatus> {
+  const res = await fetchWithRefresh("/api/checkin", {
+    method: "POST",
+    headers: clientHeaders(),
+  });
+  const data = await res.json().catch(() => ({}));
+  if (!res.ok) throw new Error(data.error || "签到失败");
+  return data as CheckinStatus;
+}