fix: enforce dd/mm/yyyy, 24h time, and locale-aware location search

- Replace all 'en-US' and undefined locales with 'en-GB' in date
  formatting across 15+ frontend files (dateUtils.ts, cards, routes,
  Luxon calls) to consistently output day-first dates and 24h times
- Set hour12: false in all Intl.DateTimeFormat and toLocaleDateString
  calls that previously used 12h format
- Pass user's svelte-i18n locale as &lang= query param from
  LocationSearchMap and LocationQuickStart to the reverse-geocode API
- Extract lang param in reverse_geocode_view and forward to both
  search_osm and search_google
- Add Accept-Language header to Nominatim requests so searches return
  results in the user's language (e.g. Prague not Praha)
- Add languageCode field to Google Places API payload for same effect

frontend/src/lib/components/cards/CollectionCard.svelte

@@ -285,12 +285,12 @@
 		<!-- Date Range (if exists) -->
 		{#if collection.start_date && collection.end_date}
 			<div class="text-xs text-base-content/60">
-				{new Date(collection.start_date).toLocaleDateString(undefined, {
+				{new Date(collection.start_date).toLocaleDateString('en-GB', {
 					timeZone: 'UTC',
 					month: 'short',
 					day: 'numeric',
 					year: 'numeric'
-				})} – {new Date(collection.end_date).toLocaleDateString(undefined, {
+				})} – {new Date(collection.end_date).toLocaleDateString('en-GB', {
 					timeZone: 'UTC',
 					month: 'short',
 					day: 'numeric',