Register + OTP Screens

Two-step wizard pages: RegisterStepPage collects email + password and triggers registerNotifier; OtpStepPage receives the email via route extra and verifies the 6-digit code, after which the session is finally persisted.

1. RegisterStepPage → registerNotifier.register(email, password).
2. On success, wizard pushes OtpStepPage with state.extra = {email}.
3. OtpStepPage submits the code → AuthApiService.verify → AuthStorageService.saveSessionOtp persists the token.
4. GoRouter redirect bounces to /onboarding_register (or /onboarding_register_seller for pending sellers).

• Hot-reload of OtpStepPage with no route extra crashes — defensive cast is intentional, dev-only failure.
• Re-sending the OTP is rate-limited server-side; UI cools down the button for ~30s without consulting the backend.
• If the user backgrounds the app between register and OTP, the partially-saved pendingRegistrationData (in AuthStorageService) keeps the wizard resumable.

• lib/src/features/shared/onboarding_register/presentation/pages/register_step_page.dart
• lib/src/features/shared/onboarding_register/presentation/pages/otp_step_page.dart
• lib/src/features/shared/onboarding_register/presentation/onboarding_register_wizard.dart

The /register and /otp routes both point into the onboarding-register wizard rather than the legacy auth/register/register.dart screen, which is kept around but unrouted.