auth: auto-complete Claude OAuth via loopback + recover onboarding on import failure

- Claude OAuth now binds a localhost callback listener (like OpenAI/Gemini)
  so the browser redirect completes the login automatically; no more
  copy/pasting the authorization code. Falls back to manual paste with the
  hosted redirect page when the loopback port is unavailable.
- On import/login failure during onboarding's Login phase, reset to the clean
  manual-login prompt instead of leaving the welcome card up (which fought the
  error message and kept the donut spinning, looking broken).
- Test: import_failure_resets_login_to_manual_prompt.