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/handler/announcement.go

@@ -0,0 +1,111 @@
+package handler
+
+import (
+	"errors"
+	"net/http"
+	"strconv"
+
+	"github.com/freefire/jiang13-bbs/service"
+	"github.com/gin-gonic/gin"
+	"gorm.io/gorm"
+)
+
+// ===== 公开接口 =====
+
+// AnnouncementsList 已发布公告列表（首页右栏 / 公告页）
+func (h *Handlers) AnnouncementsList(c *gin.Context) {
+	list, err := h.Announcement.ListPublished(10)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "获取公告失败"})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"announcements": list})
+}
+
+// AnnouncementDetail 已发布公告详情
+func (h *Handlers) AnnouncementDetail(c *gin.Context) {
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "无效的公告 ID"})
+		return
+	}
+	a, err := h.Announcement.GetPublished(uint(id))
+	if err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			c.JSON(http.StatusNotFound, gin.H{"error": "公告不存在"})
+			return
+		}
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "获取公告失败"})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"announcement": a})
+}
+
+// ===== 管理接口（RequireAdmin 兜底，前端不做权限判定） =====
+
+// AdminListAnnouncements 全部公告（含草稿）
+func (h *Handlers) AdminListAnnouncements(c *gin.Context) {
+	list, err := h.Announcement.ListAll()
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "获取公告失败"})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"announcements": list})
+}
+
+// AdminCreateAnnouncement 新建公告
+func (h *Handlers) AdminCreateAnnouncement(c *gin.Context) {
+	var in service.AnnouncementInput
+	if err := c.ShouldBindJSON(&in); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "请求参数无效"})
+		return
+	}
+	a, err := h.Announcement.Create(&in)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+	c.JSON(http.StatusCreated, gin.H{"announcement": a})
+}
+
+// AdminUpdateAnnouncement 编辑公告
+func (h *Handlers) AdminUpdateAnnouncement(c *gin.Context) {
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "无效的公告 ID"})
+		return
+	}
+	var in service.AnnouncementInput
+	if err := c.ShouldBindJSON(&in); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "请求参数无效"})
+		return
+	}
+	a, err := h.Announcement.Update(uint(id), &in)
+	if err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			c.JSON(http.StatusNotFound, gin.H{"error": "公告不存在"})
+			return
+		}
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"announcement": a})
+}
+
+// AdminDeleteAnnouncement 删除公告
+func (h *Handlers) AdminDeleteAnnouncement(c *gin.Context) {
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "无效的公告 ID"})
+		return
+	}
+	if err := h.Announcement.Delete(uint(id)); err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			c.JSON(http.StatusNotFound, gin.H{"error": "公告不存在"})
+			return
+		}
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "删除失败"})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"ok": true})
+}