Guard App.Dispose() against double-invocation and late shutdown (#1499)

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

DiscordChatExporter.Gui/App.axaml.cs

@@ -26,6 +26,8 @@ public class App : Application, IDisposable
     private readonly SettingsService _settingsService;
     private readonly MainViewModel _mainViewModel;
 
+    private bool _isDisposed;
+
     public App()
     {
         var services = new ServiceCollection();
@@ -98,8 +100,24 @@ public class App : Application, IDisposable
     public override void OnFrameworkInitializationCompleted()
     {
         if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
             desktop.MainWindow = new MainView { DataContext = _mainViewModel };
 
+            void OnExit(object? sender, ControlledApplicationLifetimeExitEventArgs args)
+            {
+                if (sender is IControlledApplicationLifetime lifetime)
+                    lifetime.Exit -= OnExit;
+
+                Dispose();
+            }
+
+            // Although `App.Dispose()` is invoked from `Program.Main(...)`, on some platforms
+            // it may be called too late in the shutdown lifecycle. Attach an exit
+            // handler to ensure timely disposal as a safeguard.
+            // https://github.com/Tyrrrz/YoutubeDownloader/issues/795
+            desktop.Exit += OnExit;
+        }
+
         base.OnFrameworkInitializationCompleted();
 
         // Set up custom theme colors
@@ -115,6 +133,11 @@ public class App : Application, IDisposable
 
     public void Dispose()
     {
+        if (_isDisposed)
+            return;
+
+        _isDisposed = true;
+
         _eventRoot.Dispose();
         _services.Dispose();
     }