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。

lib/common/widgets/notification_permission_tip.dart

@@ -73,10 +73,26 @@ class NotificationPermissionTip extends StatelessWidget {
   }
 }
 
-Future<void> showNotificationPermissionTip(BuildContext context) async {
+/// Keeps the explanation visible while Android's system permission dialog is
+/// presented. Call [remove] after the permission request completes.
+NotificationPermissionTipHandle showNotificationPermissionTip(
+  BuildContext context,
+) {
   final overlay = Overlay.of(context, rootOverlay: true);
   final entry = OverlayEntry(builder: (_) => const NotificationPermissionTip());
   overlay.insert(entry);
-  await Future<void>.delayed(const Duration(milliseconds: 500));
-  entry.remove();
+  return NotificationPermissionTipHandle._(entry);
+}
+
+class NotificationPermissionTipHandle {
+  NotificationPermissionTipHandle._(this._entry);
+
+  final OverlayEntry _entry;
+  bool _isRemoved = false;
+
+  void remove() {
+    if (_isRemoved) return;
+    _isRemoved = true;
+    _entry.remove();
+  }
 }