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/__tests__/lib/utils.test.ts

@@ -212,3 +212,14 @@ describe('computeYBounds', () => {
     expect(computeYBounds([], 'progress')).toEqual({});
   });
 });
+
+describe('isProposedToClose', () => {
+  const { isProposedToClose } = require('@/lib/utils');
+
+  it('is true only for the exact GIAS proposed-to-close status', () => {
+    expect(isProposedToClose({ status: 'Open, but proposed to close' })).toBe(true);
+    expect(isProposedToClose({ status: 'Open' })).toBe(false);
+    expect(isProposedToClose({ status: null })).toBe(false);
+    expect(isProposedToClose({})).toBe(false);
+  });
+});