fix：推送权限弹窗时序问题

lib/common/push/notification_permission.dart

@@ -23,9 +23,15 @@ abstract final class NotificationPermission {
       return isGranted();
     }
     if (await isGranted() || !context.mounted) return true;
-    await showNotificationPermissionTip(context);
+    final tip = showNotificationPermissionTip(context);
     await StorageService.to.setBool(storagePushPermissionPrompted, true);
-    return (await Permission.notification.request()).isGranted;
+    try {
+      // Match the legacy app's persistent top snackbar: keep the in-app
+      // explanation on screen until the system authorization dialog closes.
+      return (await Permission.notification.request()).isGranted;
+    } finally {
+      tip.remove();
+    }
   }
 
   /// 设置页不使用顶部说明；iOS/Android 都直接请求系统授权。