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/locations/LocationVisits.svelte

@@ -147,17 +147,17 @@
 	// Helper functions
 	function formatDateInTimezone(utcDate: string, timezone: string): string {
 		try {
-			return new Intl.DateTimeFormat(undefined, {
+			return new Intl.DateTimeFormat('en-GB', {
 				timeZone: timezone,
 				year: 'numeric',
 				month: 'short',
 				day: 'numeric',
 				hour: '2-digit',
 				minute: '2-digit',
-				hour12: true
+				hour12: false
 			}).format(new Date(utcDate));
 		} catch {
-			return new Date(utcDate).toLocaleString();
+			return new Date(utcDate).toLocaleString('en-GB');
 		}
 	}
 
@@ -957,8 +957,8 @@
 														{formatDateInTimezone(visit.start_date, visit.timezone)}
 														– {formatDateInTimezone(visit.end_date, visit.timezone)}
 													{:else}
-														{new Date(visit.start_date).toLocaleString()}
-														– {new Date(visit.end_date).toLocaleString()}
+														{new Date(visit.start_date).toLocaleString('en-GB')}
+														– {new Date(visit.end_date).toLocaleString('en-GB')}
 													{/if}
 												</div>
 											</div>