test(tui): golden state-space walker for onboarding welcome screens

Renders every OnboardingPhase to an offscreen TestBackend and asserts the
exact rendered copy, so onboarding wording can be reviewed and regression-
guarded without manually walking the live first-run flow. Run with
--nocapture to dump every screen.

Adds a #[cfg(test)] draw_onboarding_welcome_for_tests helper in ui.rs so
the test exercises the real render path without poking the private
onboarding submodule.