Register Use Case

Creates a new account from a registration payload (email, password, displayName) and returns the freshly-issued User on success.

1. Controller builds RegisterParams from the form.
2. Use case enforces password strength + email format.
3. Calls AuthRepository.register(payload).
4. Repository POSTs to /auth/signup, caches the returned token and user.
5. Returns Either<AuthFailure, User>.

• Email already taken → Left(AuthFailure.emailInUse) (HTTP 409).
• Server-side validation rejection → Left(AuthFailure.serverValidation(messages)).
• Network failure after token issued but before cache write — handled idempotently on next boot.

• lib/features/auth/domain/usecases/register_usecase.dart

Mirrors LoginUseCase intentionally — same shape, same return type — so the controller can reuse the same Either-handling helper.