feat(ui): chromeless three-island shell

src/ClaudeDo.App/App.axaml.cs

@@ -22,7 +22,7 @@ public partial class App : Application
         {
             desktop.MainWindow = new MainWindow
             {
-                DataContext = Services.GetRequiredService<MainWindowViewModel>(),
+                DataContext = Services.GetRequiredService<IslandsShellViewModel>(),
             };
         }