Implement AdminUsersScreen with create/delete user and admin gating (db)

- Add AdminUsersScreen with user list, create dialog, delete confirmation,
  and optimistic UI (revert on error) for both create and delete operations.
- Add currentUserProvider (FutureProvider) to resolve the logged-in User
  object from token storage + listUsers, used for self-delete gating and
  Settings admin section visibility.
- Gate Admin section (Manage Users tile) in SettingsScreen behind
  currentUserProvider → isAdmin, providing defence-in-depth alongside
  server-side 403 enforcement.
- Add adminUserErrorMessage to error_mappers.dart with 400/403/409 handling.
- Add adminUsers route constant (AppRoutes.adminUsers) and GoRouter entry.
- Add 25 tests in admin_users_screen_test.dart and 3 admin-section tests
  in settings_screen_test.dart (397 tests total pass).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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