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/service/announcement.go

@@ -0,0 +1,149 @@
+package service
+
+import (
+	"errors"
+	"strings"
+
+	"github.com/freefire/jiang13-bbs/model"
+	"gorm.io/gorm"
+)
+
+// 公告标签允许使用的预设色（服务端白名单，禁止透传任意颜色值）
+var allowedTagColors = map[string]bool{
+	"blue": true, "green": true, "orange": true,
+	"red": true, "purple": true, "gray": true,
+}
+
+// AnnouncementService 站点公告服务
+type AnnouncementService struct {
+	db *gorm.DB
+}
+
+func NewAnnouncementService(db *gorm.DB) *AnnouncementService {
+	return &AnnouncementService{db: db}
+}
+
+// AnnouncementInput 创建/更新公告的入参
+type AnnouncementInput struct {
+	Title     string `json:"title"`
+	Content   string `json:"content"`
+	Tag       string `json:"tag"`
+	TagColor  string `json:"tag_color"`
+	Published *bool  `json:"published"`
+}
+
+// normalize 清洗并校验入参，返回可直接落库的字段
+func (in *AnnouncementInput) normalize(existing *model.Announcement) error {
+	in.Title = strings.TrimSpace(in.Title)
+	if in.Title == "" {
+		return errors.New("标题不能为空")
+	}
+	if len([]rune(in.Title)) > 200 {
+		return errors.New("标题不能超过 200 字")
+	}
+	in.Content = strings.TrimSpace(in.Content)
+	if in.Content == "" {
+		return errors.New("内容不能为空")
+	}
+
+	in.Tag = strings.TrimSpace(in.Tag)
+	if in.Tag == "" {
+		in.Tag = "公告"
+	}
+	if len([]rune(in.Tag)) > 32 {
+		return errors.New("标签不能超过 32 字")
+	}
+
+	in.TagColor = strings.ToLower(strings.TrimSpace(in.TagColor))
+	if in.TagColor == "" {
+		if existing != nil && existing.TagColor != "" {
+			in.TagColor = existing.TagColor
+		} else {
+			in.TagColor = "blue"
+		}
+	}
+	if !allowedTagColors[in.TagColor] {
+		return errors.New("不支持的标签颜色")
+	}
+	return nil
+}
+
+// applyTo 把入参写入模型（Published 为指针以区分"未传"与"显式 false"）
+func (in *AnnouncementInput) applyTo(a *model.Announcement) {
+	a.Title = in.Title
+	a.Content = in.Content
+	a.Tag = in.Tag
+	a.TagColor = in.TagColor
+	if in.Published != nil {
+		a.Published = *in.Published
+	}
+}
+
+// ListPublished 已发布公告（首页右栏 / 公开列表），按发布时间倒序
+func (s *AnnouncementService) ListPublished(limit int) ([]model.Announcement, error) {
+	if limit <= 0 {
+		limit = 5
+	}
+	var list []model.Announcement
+	err := s.db.Where("published = ?", true).
+		Order("created_at DESC, id DESC").Limit(limit).Find(&list).Error
+	return list, err
+}
+
+// GetPublished 取单条已发布公告（详情页）；草稿/不存在返回 gorm.ErrRecordNotFound
+func (s *AnnouncementService) GetPublished(id uint) (*model.Announcement, error) {
+	var a model.Announcement
+	err := s.db.Where("published = ?", true).First(&a, id).Error
+	if err != nil {
+		return nil, err
+	}
+	return &a, nil
+}
+
+// ListAll 管理后台：含草稿，按更新时间倒序
+func (s *AnnouncementService) ListAll() ([]model.Announcement, error) {
+	var list []model.Announcement
+	err := s.db.Order("created_at DESC, id DESC").Limit(100).Find(&list).Error
+	return list, err
+}
+
+// Create 创建公告
+func (s *AnnouncementService) Create(in *AnnouncementInput) (*model.Announcement, error) {
+	if err := in.normalize(nil); err != nil {
+		return nil, err
+	}
+	a := &model.Announcement{Published: true}
+	in.applyTo(a)
+	if err := s.db.Create(a).Error; err != nil {
+		return nil, err
+	}
+	return a, nil
+}
+
+// Update 更新公告
+func (s *AnnouncementService) Update(id uint, in *AnnouncementInput) (*model.Announcement, error) {
+	var a model.Announcement
+	if err := s.db.First(&a, id).Error; err != nil {
+		return nil, err
+	}
+	if err := in.normalize(&a); err != nil {
+		return nil, err
+	}
+	in.applyTo(&a)
+	if err := s.db.Save(&a).Error; err != nil {
+		return nil, err
+	}
+	return &a, nil
+}
+
+// Delete 删除公告（软删除）
+func (s *AnnouncementService) Delete(id uint) error {
+	result := s.db.Delete(&model.Announcement{}, id)
+	if result.Error != nil {
+		return result.Error
+	}
+	if result.RowsAffected == 0 {
+		return gorm.ErrRecordNotFound
+	}
+	return nil
+}