feat(users): show last-access timestamp per user

Maps Navidrome's lastAccessAt onto NdUser and renders it in the
User Management row as a localized relative time (Intl.RelativeTimeFormat
with the current i18n language). Tooltip carries the absolute
timestamp. Navidrome returns 0001-01-01 for never-accessed users —
detected and shown as the localized "Never" label.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/locales/nl.ts

@@ -478,6 +478,7 @@ export const nlTranslation = {
     userMgmtLibrariesValidation: 'Selecteer ten minste één bibliotheek.',
     userMgmtLibrariesUpdateError: 'Gebruiker opgeslagen, maar bibliotheektoewijzing mislukt',
     userMgmtNoLibraries: 'Geen bibliotheken toegewezen',
+    userMgmtNeverSeen: 'Nooit',
     userMgmtSave: 'Opslaan',
     userMgmtCancel: 'Annuleren',
     userMgmtDelete: 'Verwijderen',