fix：推送权限弹窗时序问题

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();
+  }
 }