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/Controls/DragGhostWindow.axaml.cs

@@ -0,0 +1,29 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Media;
+
+namespace ClaudeDo.Ui.Views.Controls;
+
+/// <summary>
+/// Borderless, transparent, topmost, click-through window that hosts the translucent drag
+/// "ghost" — a snapshot of the row being dragged. It never activates (so the source window
+/// keeps pointer capture) and is repositioned to the screen cursor on every captured move.
+/// </summary>
+public partial class DragGhostWindow : Window
+{
+    public DragGhostWindow() => InitializeComponent();
+
+    /// <summary>
+    /// Show <paramref name="image"/> at <paramref name="logicalWidth"/>×<paramref name="logicalHeight"/>
+    /// with <paramref name="pad"/> of slack around it so the tilt isn't clipped by the window bounds.
+    /// </summary>
+    public void SetImage(IImage image, double logicalWidth, double logicalHeight, double pad)
+    {
+        GhostImage.Source = image;
+        GhostImage.Width = logicalWidth;
+        GhostImage.Height = logicalHeight;
+        GhostImage.Margin = new Thickness(pad);
+        Width = logicalWidth + pad * 2;
+        Height = logicalHeight + pad * 2;
+    }
+}