Fix CancellationTokenSource leak: wire ChatViewModel.Dispose() to app shutdown

[Copilot]

ChatViewModel implements IDisposable to cancel and dispose _pollCts (background connection poller) and _chatCts (active chat request), but Dispose() was never called — leaving both CancellationTokenSource instances open for the lifetime of the process.

Changes

• MainWindowViewModel — implements IDisposable; Dispose() delegates to ChatViewModel?.Dispose(), nulls the reference, and unsubscribes from ConfigService.Instance.ConfigChanged
• App.axaml.cs — hooks desktop.ShutdownRequested to prepare the window service for shutdown, and desktop.Exit via a dedicated OnExit handler that comprehensively disposes OllamaService, TrayService, ShortcutService, and MainWindowViewModel (which in turn disposes ChatViewModel)

// App.axaml.cs — OnExit handler
private void OnExit(object? sender, ControlledApplicationLifetimeExitEventArgs e)
{
    OllamaService.Instance.Dispose();
    TrayService.Instance.Dispose();
    ShortcutService.Instance?.Dispose();
    _mainWindowViewModel?.Dispose();

    if (ConfigService.Instance.GetConfig().Privacy.ClearOnClose)
        ConversationStore.Clear();
}

// MainWindowViewModel.cs
public void Dispose()
{
    ConfigService.Instance.ConfigChanged -= OnConfigChanged;
    ChatViewModel?.Dispose();
    ChatViewModel = null;
    WebBrowserViewModel = null;
}

[Copilot]

Pull request overview

Ensures ChatViewModel.Dispose() is invoked during application shutdown so its background CancellationTokenSource instances don’t remain alive for the lifetime of the process.

Changes:

• Add IDisposable to MainWindowViewModel and forward disposal to ChatViewModel.
• Hook Avalonia desktop lifetime Exit event to dispose the MainWindowViewModel on app shutdown.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
ViewModels/MainWindowViewModel.cs	Implements IDisposable and disposes/releases ChatViewModel on shutdown.
App.axaml.cs	Calls MainWindowViewModel.Dispose() from desktop.Exit to guarantee cleanup at application exit.

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[murapadev]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merged origin/main into this branch and resolved the conflict in App.axaml.cs (commit f13d616).

Main had already evolved to include a comprehensive OnExit handler that calls _mainWindowViewModel?.Dispose() (along with other service cleanup), which is a superset of our inline lambda. Kept main's version and removed the duplicate Dispose method that resulted from the merge. Build passes with 0 warnings and 0 errors.

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