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>