feat(ui): mark proposed-to-close schools in listings and detail pages

Amber tag in listing rows (option A) and a slim notice strip under the
detail-page header (option E): proposed for closure, formal process not
necessarily started, check with the local authority before applying.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

nextjs-app/lib/utils.ts

@@ -718,3 +718,18 @@ export function buildOfstedListBadge(school: {
 
   return { label: 'Not yet inspected', cssClass: 'ofstedPending' };
 }
+
+// ============================================================================
+// Establishment status
+// ============================================================================
+
+export const PROPOSED_TO_CLOSE_STATUS = 'Open, but proposed to close';
+
+/**
+ * GIAS lists some operating schools as "Open, but proposed to close".
+ * They remain open (and may stay open if the proposal is withdrawn), but the
+ * UI marks them so families check with the local authority before applying.
+ */
+export function isProposedToClose(school: { status?: string | null }): boolean {
+  return school.status === PROPOSED_TO_CLOSE_STATUS;
+}