fix: align useTableSort with PHP version — userClicked ref, nullable activeSort

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/admin/components/SortIcon.tsx

@@ -1,6 +1,6 @@
 interface SortIconProps {
   column: string
-  sort: string
+  sort: string | null
   order: string
 }