feat(ui): sixth-form badge, note and filter labels use GIAS flag

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

nextjs-app/components/SecondarySchoolRow.tsx

@@ -23,7 +23,8 @@ function detectAdmissionsTag(school: School): string | null {
 }
 
 function hasSixthForm(school: School): boolean {
-  return school.age_range?.includes('18') ?? false;
+  // GIAS OfficialSixthForm flag; missing (pipeline not yet re-run) => false.
+  return school.has_sixth_form ?? false;
 }
 
 interface SecondarySchoolRowProps {