tui: onboarding Login phase detects and offers external-login imports

The in-TUI Login welcome card now inspects importable external logins
(Codex/Claude/Cursor/Copilot/Gemini/shared sources) at startup. When any
are found it lists them on the card and arms the AutoImportSelection
prompt so the user can type 'a' (all) or '1,3' to import; otherwise it
falls back to 'press Enter to choose a provider'.

- OnboardingPhase::Login now carries detected_imports: Vec<String>
- OnboardingWelcomeKind::Login mirrors it for rendering
- begin_external_auth_import_review(announce) skips the transcript push
  during onboarding (welcome card renders the list instead)