fix(search): correct radius units and distance display for postcode search

FilterBar was sending radius in km (e.g. 1.6) but the backend expects miles,
causing the "Showing schools within X miles" banner to display the wrong value.
Change option values to miles (0.5, 1, 3, 5, 10) and default from 1.6 to 1.

school.distance from the API is already in miles (backend haversine uses
R=3959). SchoolRow was dividing by 1609.34 giving 0.0 mi; CompactSchoolItem
was dividing by 1.60934. Both now display school.distance directly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nextjs-app/components/SchoolRow.tsx

@@ -108,7 +108,7 @@ export function SchoolRow({
             )}
           {isLocationSearch && school.distance != null && (
             <span className={styles.distanceBadge}>
-              {(school.distance / 1609.34).toFixed(1)} mi
+              {school.distance.toFixed(1)} mi
             </span>
           )}
         </div>