Add OnboardingProviderProtocol so modules declare their own post-signup
onboarding steps. The core OnboardingAggregator discovers enabled
providers and exposes a dashboard API (GET /dashboard/onboarding).
A session-scoped banner on the store dashboard shows a checklist that
guides merchants through setup without blocking signup.
Signup is simplified from 4 steps to 3 (Plan → Account → Payment):
store creation is merged into account creation, store language is
captured from the user's browsing language, and platform-specific
template branching is removed.
Includes 47 unit and integration tests covering all new providers,
the aggregator, the API endpoint, and the signup service changes.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Samir Boulahtit
ef9ea29643