Bind notification popup mask to the toast column item

Use Region { item: popupColumn } instead of manually mirroring the
column's x/y/width/height. It tracks the item's mapped geometry directly,
so it stays correct without assuming the column shares the window's
coordinate space.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

shell/plugins/notifications/Service.qml

@@ -795,12 +795,7 @@ Item {
 
       // Keep the surface click-through except over the toast column, so the
       // rest of the (invisible) full-screen overlay never eats input.
-      mask: Region {
+      mask: Region { item: popupColumn }
-        x: popupColumn.x
-        y: popupColumn.y
-        width: popupColumn.width
-        height: popupColumn.height
-      }
 
       ColumnLayout {
         id: popupColumn