fix(U4): guard showNewMailNotification to Android-only to prevent DBus hang on Linux

lib/core/services/notification_service.dart

@@ -1,3 +1,5 @@
+import 'dart:io';
+
 import 'package:flutter_local_notifications/flutter_local_notifications.dart';
 
 const _kChannelId = 'new_mail';
@@ -18,6 +20,7 @@ Future<void> initNotifications() async {
 }
 
 Future<void> showNewMailNotification(String accountEmail) async {
+  if (!Platform.isAndroid) return;
   await _plugin.show(
     accountEmail.hashCode & 0x7FFFFFFF,
     'New mail',