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/Design/IslandStyles.axaml

@@ -407,6 +407,8 @@
         <BrushTransition     Property="Background"  Duration="0:0:0.12" />
         <BrushTransition     Property="BorderBrush" Duration="0:0:0.12" />
         <ThicknessTransition Property="Margin"      Duration="0:0:0.15" />
+        <TransformOperationsTransition Property="RenderTransform" Duration="0:0:0.12" />
+        <DoubleTransition    Property="Opacity"     Duration="0:0:0.12" />
       </Transitions>
     </Setter>
   </Style>
@@ -417,6 +419,13 @@
     <Setter Property="BorderBrush"     Value="{StaticResource AccentBrush}" />
     <Setter Property="BorderThickness" Value="1" />
   </Style>
+  <!-- "Grabbed" row: lift + slight scale + lower opacity + shadow while the custom drag runs. -->
+  <Style Selector="Border.task-row.dragging">
+    <Setter Property="Opacity"         Value="0.55" />
+    <Setter Property="RenderTransform" Value="scale(1.03)" />
+    <Setter Property="BoxShadow"       Value="0 10 26 0 #66000000" />
+    <Setter Property="BorderBrush"     Value="{StaticResource AccentBrush}" />
+  </Style>
 
   <!-- Checkbox indicator (the 18px circle that replaces the native CheckBox template) -->
   <Style Selector="Ellipse.task-check">