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/lib/utils.ts

@@ -235,6 +235,9 @@ export function getVariabilityLabel(stdDev: number | null): string {
 // Validation
 // ============================================================================
 
+/** Radius a postcode search uses until the user picks another. */
+export const DEFAULT_RADIUS_MILES = 0.5;
+
 /**
  * Validate UK postcode format
  */