fix(ui): add vertical spacing between task rows

Cards touched with Margin=0. Add margin on the inner task-row border
instead of the ListBoxItem style, so the drag/drop hit-test container
still covers the row and the drop-hint doesn't flicker between items.

src/ClaudeDo.Ui/Views/Islands/TaskRowView.axaml

@@ -29,7 +29,7 @@
 
       <!-- Main task card -->
       <Border Grid.Column="1" Classes="task-row"
-              Margin="0"
+              Margin="0,2"
               Classes.selected="{Binding IsSelected}"
               Classes.dragging="{Binding IsDragging}"
               Classes.drop-target="{Binding IsDropTarget}"