feat: 新增站点公告系统与多项体验优化

此提交完成了以下核心功能与优化：
1. 新增站点公告全流程支持：包含公告模型、CRUD接口、管理后台页面、公告详情页与首页展示
2. 实现评论楼层化与回复嵌套：重构评论系统为树状结构，支持回复评论与层级限制
3. 添加用户在线状态追踪：异步更新用户最后活跃时间并做SQL限频
4. 优化用户体验：移动端浮动操作条、统一板块标签样式、用户页签到积分展示、注册即登录优化
5. 其他细节优化：SEO站点地图新增公告页、评论分页与统计、登录/注册表单增强等

backend/router/router.go

@@ -45,6 +45,7 @@ func Setup(cfg *config.Config) (*gin.Engine, error) {
 	notifSvc := service.NewNotificationService(model.DB)
 	overviewSvc := service.NewOverviewService(model.DB)
 	checkinSvc := service.NewCheckinService(model.DB)
+	announcementSvc := service.NewAnnouncementService(model.DB)
 	limiter := service.DefaultRateLimiter()
 
 	h := &handler.Handlers{
@@ -57,6 +58,7 @@ func Setup(cfg *config.Config) (*gin.Engine, error) {
 		Notification: notifSvc,
 		OverviewSvc:  overviewSvc,
 		Checkin:      checkinSvc,
+		Announcement: announcementSvc,
 	}
 
 	authMW := middleware.NewAuthMiddleware(authSvc)
@@ -77,6 +79,8 @@ func Setup(cfg *config.Config) (*gin.Engine, error) {
 		pubAPI.GET("/posts/:id/comments", h.PostComments)
 		pubAPI.GET("/users/:id", h.UserProfile)
 		pubAPI.GET("/users/:id/comments", h.UserComments)
+		pubAPI.GET("/announcements", h.AnnouncementsList)
+		pubAPI.GET("/announcements/:id", h.AnnouncementDetail)
 		pubAPI.POST("/register", middleware.RateLimitMiddleware(limiter, service.RateRegister), h.Register)
 		pubAPI.POST("/login", middleware.RateLimitMiddleware(limiter, service.RateLogin), h.Login)
 		// refresh token 端点：access 过期后用 refresh 换新 token（需 CSRF 防护）
@@ -112,6 +116,11 @@ func Setup(cfg *config.Config) (*gin.Engine, error) {
 		adminAPI.GET("/dashboard", func(c *gin.Context) {
 			c.JSON(200, gin.H{"message": "admin dashboard"})
 		})
+		// 站点公告文章管理
+		adminAPI.GET("/announcements", h.AdminListAnnouncements)
+		adminAPI.POST("/announcements", h.AdminCreateAnnouncement)
+		adminAPI.PUT("/announcements/:id", h.AdminUpdateAnnouncement)
+		adminAPI.DELETE("/announcements/:id", h.AdminDeleteAnnouncement)
 	}
 
 	r.NoRoute(func(c *gin.Context) {