fix: use user locale instead of hardcoded de-DE for number/date formatting — closes #144

- CurrencyWidget: format numbers with user's locale
- ReservationModal: date formatting uses locale
- TripPDF: locale fallback to browser default instead of de-DE
- holidays.ts: formatDate accepts optional locale parameter

client/src/components/PDF/TripPDF.tsx

@@ -110,7 +110,7 @@ interface downloadTripPDFProps {
 
 export async function downloadTripPDF({ trip, days, places, assignments, categories, dayNotes, reservations = [], t: _t, locale: _locale }: downloadTripPDFProps) {
   await ensureRenderer()
-  const loc = _locale || 'de-DE'
+  const loc = _locale || undefined
   const tr = _t || (k => k)
   const sorted = [...(days || [])].sort((a, b) => a.day_number - b.day_number)
   const range = longDateRange(sorted, loc)