Continue directly to login after customer signup

[kcrmin]

What changed

• read verificationRequired and nullable verification expiry from signup responses
• redirect immediately to login when verification is not required
• show a clear signup-complete message
• retain the existing verification screen for a future backend response that requires verification

Why

The current no-SES development environment creates login-ready customer accounts, so the UI must not strand users on an email-delivery screen.

Validation

• pnpm typecheck
• 49 test files / 588 tests passed
• pnpm build
• live Docker signup and immediate login passed against the updated backend

Depends on the matching backend PR.