Fix desktop close listener callback signature

lib/services/window_lifecycle_service.dart

@@ -1,4 +1,5 @@
 import 'dart:io';
+import 'dart:async';
 
 import 'package:window_manager/window_manager.dart';
 
@@ -29,7 +30,11 @@ class WindowLifecycleService with WindowListener {
   }
 
   @override
-  Future<void> onWindowClose() async {
+  void onWindowClose() {
+    unawaited(_handleWindowClose());
+  }
+
+  Future<void> _handleWindowClose() async {
     if (_isClosing) {
       return;
     }