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.

tests/ClaudeDo.Ui.Tests/DragHitTestTests.cs

@@ -0,0 +1,44 @@
+using Avalonia;
+using ClaudeDo.Ui.Views.Controls;
+using Xunit;
+
+namespace ClaudeDo.Ui.Tests;
+
+public class DragHitTestTests
+{
+    [Fact]
+    public void Cursor_inside_window_rect_is_contained()
+    {
+        var pos = new PixelPoint(100, 100);
+        var client = new Size(800, 600);
+        Assert.True(DragHitTest.WindowContains(pos, client, 1.0, new PixelPoint(500, 400)));
+    }
+
+    [Fact]
+    public void Cursor_outside_window_rect_is_not_contained()
+    {
+        var pos = new PixelPoint(100, 100);
+        var client = new Size(800, 600);
+        Assert.False(DragHitTest.WindowContains(pos, client, 1.0, new PixelPoint(50, 50)));   // above-left
+        Assert.False(DragHitTest.WindowContains(pos, client, 1.0, new PixelPoint(950, 400))); // past right edge
+    }
+
+    [Fact]
+    public void Top_left_is_inclusive_bottom_right_is_exclusive()
+    {
+        var pos = new PixelPoint(100, 100);
+        var client = new Size(800, 600);
+        Assert.True(DragHitTest.WindowContains(pos, client, 1.0, new PixelPoint(100, 100)));   // top-left corner
+        Assert.False(DragHitTest.WindowContains(pos, client, 1.0, new PixelPoint(900, 700)));  // bottom-right corner
+    }
+
+    [Fact]
+    public void Scaling_expands_the_physical_rect()
+    {
+        var pos = new PixelPoint(100, 100);
+        var client = new Size(800, 600); // logical
+        // At 2x the right edge is 100 + 1600 = 1700, so a point at x=1600 now falls inside.
+        Assert.False(DragHitTest.WindowContains(pos, client, 1.0, new PixelPoint(1600, 400)));
+        Assert.True(DragHitTest.WindowContains(pos, client, 2.0, new PixelPoint(1600, 400)));
+    }
+}