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/MainWindow.axaml.cs

@@ -1,6 +1,7 @@
 using Avalonia.Controls;
 using Avalonia.Input;
 using ClaudeDo.Ui.ViewModels;
+using ClaudeDo.Ui.Views.Modals;
 using ClaudeDo.Ui.Views.Planning;
 
 namespace ClaudeDo.Ui.Views;
@@ -23,6 +24,13 @@ public partial class MainWindow : Window
                 var modal = new ConflictResolutionView { DataContext = conflictVm };
                 await modal.ShowDialog(this);
             };
+            vm.ShowAboutModal = async (aboutVm) =>
+            {
+                var dlg = new AboutModalView { DataContext = aboutVm };
+                var tcs = new TaskCompletionSource<bool>();
+                aboutVm.CloseAction = () => { dlg.Close(); tcs.TrySetResult(true); };
+                await dlg.ShowDialog(this);
+            };
         }
     }