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/en.ts

@@ -491,6 +491,7 @@ export const enTranslation = {
     userMgmtLibrariesValidation: 'Select at least one library.',
     userMgmtLibrariesUpdateError: 'User saved, but library assignment failed',
     userMgmtNoLibraries: 'No libraries assigned',
+    userMgmtNeverSeen: 'Never',
     userMgmtSave: 'Save',
     userMgmtCancel: 'Cancel',
     userMgmtDelete: 'Delete',