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

@@ -161,6 +161,58 @@
           </StackPanel>
         </Border>
 
+        <!-- Attachments section -->
+        <Border BorderBrush="{DynamicResource LineBrush}"
+                BorderThickness="0,1,0,0"
+                Padding="0,8,0,0">
+          <StackPanel Spacing="6">
+            <TextBlock Classes="section-label" Text="{loc:Tr details.attachments.sectionLabel}"/>
+
+            <!-- Attachment rows -->
+            <ItemsControl ItemsSource="{Binding Attachments}">
+              <ItemsControl.ItemTemplate>
+                <DataTemplate DataType="vm:AttachmentRowViewModel">
+                  <Grid ColumnDefinitions="*,Auto,Auto" Margin="0,2,0,2">
+                    <TextBlock Grid.Column="0"
+                               Text="{Binding FileName}"
+                               VerticalAlignment="Center"
+                               FontSize="{StaticResource FontSizeBody}"
+                               Foreground="{DynamicResource TextDimBrush}"
+                               TextTrimming="CharacterEllipsis"/>
+                    <TextBlock Grid.Column="1"
+                               Text="{Binding SizeText}"
+                               VerticalAlignment="Center"
+                               Margin="8,0"
+                               FontSize="{StaticResource FontSizeBody}"
+                               Foreground="{DynamicResource TextMuteBrush}"/>
+                    <Button Grid.Column="2"
+                            Classes="icon-btn"
+                            ToolTip.Tip="{loc:Tr details.attachments.removeTip}"
+                            Command="{Binding $parent[ItemsControl].((vm:DetailsIslandViewModel)DataContext).RemoveAttachmentCommand}"
+                            CommandParameter="{Binding}">
+                      <PathIcon Data="{StaticResource Icon.X}" Width="11" Height="11"/>
+                    </Button>
+                  </Grid>
+                </DataTemplate>
+              </ItemsControl.ItemTemplate>
+            </ItemsControl>
+
+            <!-- Add file button -->
+            <Button Classes="btn"
+                    Padding="8,3"
+                    HorizontalAlignment="Left"
+                    Click="OnAddFileClick"
+                    Content="{loc:Tr details.attachments.addFile}"/>
+
+            <!-- Drop status / confirmation -->
+            <TextBlock Text="{Binding DropStatus}"
+                       IsVisible="{Binding DropStatus, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
+                       FontSize="{StaticResource FontSizeBody}"
+                       Foreground="{DynamicResource TextMuteBrush}"
+                       TextWrapping="Wrap"/>
+          </StackPanel>
+        </Border>
+
       </StackPanel>
       </ScrollViewer>
     </DockPanel>

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();
+        }
+    }
 }