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/es.ts

@@ -482,6 +482,7 @@ export const esTranslation = {
     userMgmtLibrariesValidation: 'Selecciona al menos una biblioteca.',
     userMgmtLibrariesUpdateError: 'Usuario guardado, pero la asignación de bibliotecas falló',
     userMgmtNoLibraries: 'Sin bibliotecas asignadas',
+    userMgmtNeverSeen: 'Nunca',
     userMgmtSave: 'Guardar',
     userMgmtCancel: 'Cancelar',
     userMgmtDelete: 'Eliminar',