feat: show date badge on day selectors + i18n transport modal titles

Day selectors in the Transport, Reservation and Hotel-Day-Range
modals only showed the renamed day title once a day had a custom
name — hiding the actual date. Added an optional badge prop to
CustomSelect, rendered as a pill next to the label, and wired the
date badge onto all affected dropdowns. FileManager day section
headers got the same pill for consistency.

Also translated transport.addTransport and transport.modalTitle.*
in all 13 non-English language files; the keys existed but still
carried the English source string.

client/src/components/Planner/TransportModal.tsx

@@ -222,7 +222,8 @@ export function TransportModal({ isOpen, onClose, onSave, reservation, days, sel
     { value: '', label: '—' },
     ...days.map(d => ({
       value: d.id,
-      label: d.title || `${t('dayplan.dayN', { n: d.day_number })}${d.date ? ` · ${formatDate(d.date, locale) ?? ''}` : ''}`,
+      label: d.title || t('dayplan.dayN', { n: d.day_number }),
+      badge: d.date ? (formatDate(d.date, locale) ?? undefined) : undefined,
     })),
   ]