feat(ui): add MarkdownView control and editable description in details island

New MarkdownView UserControl renders a markdown preview. Details island gains an editable Description section with edit/preview toggle, collapsible header, copy-to-clipboard, and debounced auto-save (400ms).

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

src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs

@@ -1,5 +1,6 @@
 using Avalonia;
 using Avalonia.Controls;
+using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
 using Avalonia.Layout;
 using Avalonia.Media;
@@ -147,4 +148,12 @@ public partial class DetailsIslandView : UserControl
         if (DataContext is DetailsIslandViewModel vm)
             vm.SaveNotesCommand.Execute(null);
     }
+
+    private async void OnCopyDescriptionClick(object? sender, RoutedEventArgs e)
+    {
+        if (DataContext is not DetailsIslandViewModel vm) return;
+        var clipboard = TopLevel.GetTopLevel(this)?.Clipboard;
+        if (clipboard is null) return;
+        await clipboard.SetTextAsync(vm.EditableDescription ?? string.Empty);
+    }
 }