fix(search): start postcode searches at half a mile

New postcode searches, the near-me button and URLs without a radius now
use 0.5 miles. A postcode URL with no radius used to show "1 mile" in the
Distance control while the API applied its own 5-mile default; the page
and the map fetch now send the same default the control displays.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

nextjs-app/app/(frontend)/page.tsx

@@ -6,7 +6,7 @@
 import { absoluteUrl } from '@/lib/site';
 import type { Metadata } from 'next';
 import { fetchSchools, fetchFilters, fetchDataInfo } from '@/lib/api';
-import { formatAcademicYear } from '@/lib/utils';
+import { DEFAULT_RADIUS_MILES, formatAcademicYear } from '@/lib/utils';
 import { HomeView } from '@/components/HomeView';
 import { getFlags } from '@/lib/flags';
 import { HowItWorksSection } from '@/components/HowItWorksSection';
@@ -71,7 +71,9 @@ export default async function HomePage({ searchParams }: HomePageProps) {
 
   // Parse search params
   const page = parseInt(params.page || '1');
-  const radius = params.radius ? parseFloat(params.radius) : undefined;
+  // Without this the API applies its own 5-mile default while the Distance
+  // control shows DEFAULT_RADIUS_MILES.
+  const radius = params.radius ? parseFloat(params.radius) : DEFAULT_RADIUS_MILES;
 
   // Check if user has performed a search
   const hasSearchParams = !!(