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/routes/transportations/[id]/+page.svelte

@@ -263,7 +263,9 @@
 			if (!dateStr || isAllDay(dateStr)) return null;
 			const dt = DateTime.fromISO(dateStr, { zone: zone ?? 'UTC' });
 			if (!dt.isValid) return null;
-			return dt.setZone(localTimeZone).toLocaleString(DateTime.DATETIME_MED);
+			return dt.setZone(localTimeZone).toLocaleString(DateTime.DATETIME_MED, {
+				locale: 'en-GB'
+			});
 		};
 
 		const startLocal = formatLocal(start, startTimezone);