feat(ui): add About modal opened from Help menu

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Modals/AboutModalView.axaml.cs

@@ -0,0 +1,10 @@
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace ClaudeDo.Ui.Views.Modals;
+
+public partial class AboutModalView : Window
+{
+    public AboutModalView() => InitializeComponent();
+    private void InitializeComponent() => AvaloniaXamlLoader.Load(this);
+}