fix(ui): port retail item drag visuals

Keep retail's underlay-free m_pDragIcon separate from the full cell icon and reveal the authored ghost mesh on physical source cells for the complete drag lifecycle. This removes the backpack backing from the cursor and retires AP-47.

Co-Authored-By: Codex <codex@openai.com>

src/AcDream.App/UI/UiElement.cs

@@ -357,6 +357,14 @@ public abstract class UiElement
     /// <see cref="UiRoot"/> item-agnostic. Retail analog: m_dragIcon (decomp 229738).</summary>
     public virtual (uint tex, int w, int h)? GetDragGhost() => null;
 
+    /// <summary>
+    /// Notifies the source widget when the root starts or finishes carrying its drag payload.
+    /// This is a retained-widget lifecycle hook rather than item-specific root logic. Retail's
+    /// item implementation uses it to show/hide <c>m_elem_Icon_Ghosted</c> around a physical
+    /// item drag (<c>ItemList_BeginDrag</c> 0x004e32d0; <c>SetWaitingState</c> 0x004e11b0).
+    /// </summary>
+    internal virtual void SetDragSourceActive(bool active, object? payload) { }
+
     /// <summary>
     /// Tooltip text for this widget. Retail fires event 0x07 after the configured
     /// hover delay (0.25 seconds by default), then queries the widget's virtual "GetString"