feat(attachments): drag-and-drop file attachments on the detail pane

Drop a file anywhere on the detail pane to attach it: pane-wide drop target
with a 'Drop to attach' hover overlay (Copy cursor, gated on an idle selected
task), an explicit lingering confirmation/error line, plus an Attachments list
with size, remove, and an Add file… picker in the DETAILS card. ComposedPreview
now shows the reference files too. en/de keys added.

src/ClaudeDo.Ui/Views/Islands/Detail/DescriptionStepsCard.axaml.cs

@@ -2,6 +2,7 @@ using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
+using Avalonia.Platform.Storage;
 using ClaudeDo.Ui.ViewModels.Islands;
 
 namespace ClaudeDo.Ui.Views.Islands.Detail;
@@ -34,4 +35,32 @@ public partial class DescriptionStepsCard : UserControl
             && vm.CommitSubtaskEditCommand.CanExecute(row))
             vm.CommitSubtaskEditCommand.Execute(row);
     }
+
+    private async void OnAddFileClick(object? sender, RoutedEventArgs e)
+    {
+        if (DataContext is not DetailsIslandViewModel vm) return;
+        var topLevel = TopLevel.GetTopLevel(this);
+        if (topLevel is null) return;
+
+        var picked = await topLevel.StorageProvider.OpenFilePickerAsync(
+            new FilePickerOpenOptions { AllowMultiple = true });
+
+        if (picked.Count == 0) return;
+
+        var files = new List<(string FileName, System.IO.Stream Content)>();
+        foreach (var item in picked)
+        {
+            var stream = await item.OpenReadAsync();
+            files.Add((item.Name, stream));
+        }
+
+        try
+        {
+            await vm.AddFilesAsync(files);
+        }
+        finally
+        {
+            foreach (var (_, s) in files) await s.DisposeAsync();
+        }
+    }
 }