feat(ui): ghost-window drag infrastructure for task rows

Add the borderless, transparent, topmost, click-through DragGhostWindow that
hosts a tilted (~-6deg) translucent snapshot of the dragged row, a
TaskDragController that owns its lifecycle (snapshot -> show -> follow -> close),
and a pure DPI-aware DragHitTest helper (unit-tested) for the cross-window
screen hit test. Adds the TaskRowViewModel.IsDragging flag and the
'grabbed' Border.task-row.dragging style (lift + scale + lower opacity +
shadow). Not yet wired into the drag source.

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

@@ -30,6 +30,7 @@
       <Border Grid.Column="1" Classes="task-row"
               Margin="0"
               Classes.selected="{Binding IsSelected}"
+              Classes.dragging="{Binding IsDragging}"
               Classes.done="{Binding Done}">
         <Border.ContextMenu>
           <ContextMenu>