feat: 站点媒体库与移动端底部导航，服务测试补强

- 新增媒体库：media_library/media_thumbs 服务（WebP 缩略图）、管理端 media 页面
- 移动端底部导航 MobileTabBar 替换 MobilePostBar
- 旧数据导入增强与测试、路由与登录会话/板块侧边栏测试补强
- site-doc 组件精简（移除 Breadcrumb），文档新增迁移公告说明

backend/service/chat.go

@@ -397,7 +397,9 @@ func (s *ChatService) GetOrCreateDM(meID, peerID uint) (*RoomView, error) {
 	var existing model.ChatRoom
 	err := s.db.Where("room_type = ? AND direct_key = ?", model.ChatRoomTypeDirect, key).First(&existing).Error
 	if err == nil {
-		_ = s.ensureMember(existing.ID, meID)
+		// 关闭私聊会硬删成员行、丢失已读水位；主动重开视为已读历史，
+		// 否则重建行 last_read=0 会把全部历史消息重算成未读（列表徽标闪现）
+		_ = s.ensureMember(existing.ID, meID, existing.LastMessageID)
 		_ = s.ensureMember(existing.ID, peerID)
 		return s.RoomDetail(meID, existing.ID, false)
 	}
@@ -431,7 +433,8 @@ func (s *ChatService) GetOrCreateDM(meID, peerID uint) (*RoomView, error) {
 	return s.RoomDetail(meID, room.ID, false)
 }
 
-func (s *ChatService) ensureMember(roomID, userID uint) error {
+// ensureMember 保证成员行存在；重建（如私聊关闭后重开）时可选恢复已读水位
+func (s *ChatService) ensureMember(roomID, userID uint, readWatermark ...uint) error {
 	_, err := s.membership(s.db, roomID, userID)
 	if err == nil {
 		return nil
@@ -440,6 +443,9 @@ func (s *ChatService) ensureMember(roomID, userID uint) error {
 		return err
 	}
 	m := model.ChatRoomMember{RoomID: roomID, UserID: userID, Role: model.ChatRoleMember}
+	if len(readWatermark) > 0 {
+		m.LastReadMessageID = readWatermark[0]
+	}
 	if e := s.db.Create(&m).Error; e != nil {
 		return e
 	}