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/ViewModels/Islands/TaskRowViewModel.cs

@@ -35,6 +35,8 @@ public sealed partial class TaskRowViewModel : ViewModelBase
     [ObservableProperty] private bool _parentInView = true;
     [ObservableProperty] private int _roadblockCount;
     [ObservableProperty] private bool _isRefining;
+    // Set by the custom drag while this row is being dragged — drives the "grabbed" row style.
+    [ObservableProperty] private bool _isDragging;
 
     public bool CanRefine => Status == TaskStatus.Idle && PlanningPhase == PlanningPhase.None && !IsRefining;