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

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

nextjs-app/lib/types.ts

@@ -17,6 +17,7 @@ export interface School {
   school_type_code: string | null;
   religious_denomination: string | null;
   age_range: string | null;
+  has_sixth_form?: boolean | null;
 
   // Address
   address1: string | null;