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/DetailsIslandView.axaml

@@ -5,7 +5,9 @@
              xmlns:detail="using:ClaudeDo.Ui.Views.Islands.Detail"
              xmlns:loc="using:ClaudeDo.Ui.Localization"
              x:Class="ClaudeDo.Ui.Views.Islands.DetailsIslandView"
-             x:DataType="vm:DetailsIslandViewModel">
+             x:DataType="vm:DetailsIslandViewModel"
+             DragDrop.AllowDrop="True">
+  <Panel>
   <DockPanel>
 
     <!-- ── Metadata footer (sticky bottom) — created-at + close — task detail only ── -->
@@ -124,4 +126,21 @@
     </Grid>
 
   </DockPanel>
+
+  <!-- Drop overlay — shown while dragging files over the pane -->
+  <Border IsVisible="{Binding IsDragOver}"
+          Background="{DynamicResource AccentSoftBrush}"
+          BorderBrush="{DynamicResource AccentBrush}"
+          BorderThickness="2"
+          CornerRadius="14"
+          IsHitTestVisible="False">
+    <TextBlock Text="{loc:Tr details.attachments.dropToAttach}"
+               HorizontalAlignment="Center"
+               VerticalAlignment="Center"
+               Foreground="{DynamicResource AccentBrush}"
+               FontSize="16"
+               FontWeight="Medium"/>
+  </Border>
+
+  </Panel>
 </UserControl>