feat(schools): label age range as "Ages 3–11" for clarity

The bare "3-11" range gave no unit. Add a display-only formatAgeRange
helper ("3-11" -> "Ages 3–11", en-dash) used in the primary/secondary
search rows and the secondary detail badge. The raw age_range field is
left untouched so sixth-form detection (.includes('18')) still works.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nextjs-app/components/SecondarySchoolDetailView.tsx

@@ -23,7 +23,7 @@ import type {
   SchoolAdmissions, SenDetail, Phonics,
   SchoolDeprivation, SchoolFinance, NationalAverages,
 } from '@/lib/types';
-import { formatPercentage, formatProgress, formatAcademicYear } from '@/lib/utils';
+import { formatPercentage, formatProgress, formatAcademicYear, formatAgeRange } from '@/lib/utils';
 import { DeltaChip } from './DeltaChip';
 import { track, getNavigationSource } from '@/lib/analytics';
 import styles from './SecondarySchoolDetailView.module.css';
@@ -228,7 +228,7 @@ export function SecondarySchoolDetailView({
                 <span className={styles.badge}>{schoolInfo.gender}&apos;s school</span>
               )}
               {schoolInfo.age_range && (
-                <span className={styles.badge}>{schoolInfo.age_range}</span>
+                <span className={styles.badge}>{formatAgeRange(schoolInfo.age_range)}</span>
               )}
               {hasSixthForm && (
                 <span className={styles.badge}>Sixth form</span>