fix(zoom): the magnifier freezes the layers — no picking up objects while it is open

While zoom is active a left drag moves the box and never selects or moves a layer beneath
it (creator ruling 2026-11-01). MouseMove already owned the zoom drag; the missing guard was
in MouseLeftButtonDown, which happily hit-tested a layer first and left a stray selection
plus a half-started drag behind the moving box.

Controls/PreviewPane.xaml.cs

@@ -483,6 +483,10 @@ public partial class PreviewPane : UserControl
             e.Handled = true;
             return;
         }
+
+        // Zoom freezes the layers (creator ruling 2026-11-01): while the magnifier is up, a left
+        // drag moves the box and NEVER picks up a layer underneath it. MouseMove owns the drag.
+        if (vm.IsZoomActive) return;
         var selected = vm.SelectedElement;
 
         if (selected is { } sel && IsDraggableElement(sel) && HitHandle(e.GetPosition(grid), sel))