feat: 后台增加开放消息开关

关闭后前台隐藏私聊/群聊入口并由后端拦截；同步去掉公告页重复计数胶囊。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/handler/auth.go

@@ -279,7 +279,12 @@ func (h *Handlers) Me(c *gin.Context) {
 	if n, err := h.Notification.UnreadCount(claims.ID); err == nil {
 		unread = n
 	}
-	chatUnread := h.Chat.TotalUnread(claims.ID)
+	chatUnread := int64(0)
+	if h.Setting != nil && h.Chat != nil {
+		if allow, err := h.Setting.AllowMessages(); err == nil && allow {
+			chatUnread = h.Chat.TotalUnread(claims.ID)
+		}
+	}
 	// 板块管理员带出授权板块，供前端菜单/按钮按板块范围渲染
 	var boardIDs []uint
 	if user.Role == model.RoleBoardAdmin {