Adds a single-select referral-source step between the goals and connect
steps of onboarding. The choice is stored on users.referral_source and
mirrored to PostHog, mirroring the existing persona and goals steps.
- ReferralSource enum (12 sources) + nullable users.referral_source column
- referralSource()/storeReferralSource() controller actions with the same
self-hosted, subscribed, persona and goals guards as the sibling steps
- connect() now requires a referral source before rendering
- Single-select ReferralSource.vue page mirroring the goals step
- Localized across all 15 locales
