The welcome overlay replayed for existing users signing in, because it
had no signal for "this user just signed up" and inferred it from a
coincidence: a COMPLETED user standing on an onboarding URL while being
redirected away.
This drops that inference and triggers explicitly at the only two places
onboarding reaches COMPLETED: `useSetNextOnboardingStatus` and the
Stripe return.
Raphaël Bosi
dd57842187