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__/components/SecondarySchoolRow.test.tsx

@@ -44,3 +44,24 @@ describe('SecondarySchoolRow sixth-form tag', () => {
     expect(screen.queryByText('Sixth form')).not.toBeInTheDocument();
   });
 });
+
+describe('SecondarySchoolRow proposed-to-close tag', () => {
+  it('shows the tag when GIAS status is "Open, but proposed to close"', () => {
+    render(
+      <SecondarySchoolRow
+        school={{ ...base, status: 'Open, but proposed to close' }}
+      />,
+    );
+    expect(screen.getByText(/Proposed to close/)).toBeInTheDocument();
+  });
+
+  it('hides the tag for a plain open school', () => {
+    render(<SecondarySchoolRow school={{ ...base, status: 'Open' }} />);
+    expect(screen.queryByText(/Proposed to close/)).not.toBeInTheDocument();
+  });
+
+  it('hides the tag when status is missing', () => {
+    render(<SecondarySchoolRow school={base} />);
+    expect(screen.queryByText(/Proposed to close/)).not.toBeInTheDocument();
+  });
+});