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/profile/[uuid]/+page.svelte

@@ -333,12 +333,12 @@
 						<div class="flex items-center justify-center gap-2 text-base-content/60">
 							<Calendar class="w-5 h-5" />
 							<span class="text-lg">
-								{$t('profile.member_since')}
-								{new Date(user.date_joined).toLocaleDateString(undefined, {
-									timeZone: 'UTC',
-									year: 'numeric',
-									month: 'long'
-								})}
+							{$t('profile.member_since')}
+							{new Date(user.date_joined).toLocaleDateString('en-GB', {
+								timeZone: 'UTC',
+								year: 'numeric',
+								month: 'long'
+							})}
 							</span>
 						</div>
 					{/if}