fix: admin user list headers never announce the current sort to screen readers

In Admin Panel > Users, the column headers kept reporting the sort order from page load to screen readers, even after clicking a header re-sorted the list. Now the clicked header reports ascending or descending and all other headers report no sorting.

Fixes #31581

src/lib/components/admin/Users/UserList.svelte

@@ -72,7 +72,7 @@
 		}
 	};
 
-	const sortState = (key) =>
+	$: sortState = (key) =>
 		orderBy === key ? (direction === 'asc' ? 'ascending' : 'descending') : 'none';
 
 	const setSortKey = (key) => {