[Feature] Remove referral code step from onboarding flow

[graycyrus]

Summary

Remove the "Referral Code" step from the onboarding overlay so users go directly from Welcome → Skills → Context Gathering.

Problem

The referral code step (Step 1 — ReferralApplyStep) adds friction to onboarding. It should be disabled/commented out. The referral feature may return later, so the component file should be preserved but unused.

Solution

Comment out / disable all referral-related logic in the onboarding flow:

• app/src/pages/onboarding/Onboarding.tsx — Remove referral prefetch (referralApi.getStats()), skipReferralStep state, resolveOnboardingStep helper, referral-related navigation logic, and case 1 render branch. Simplify step numbering: Welcome (0) → Skills (1) → Context Gathering (2).
• app/src/pages/onboarding/steps/ReferralApplyStep.tsx — Keep file (unused), no changes needed.
• Referral API (referralApi) stays untouched since it's used elsewhere (e.g. Rewards page).

Acceptance criteria

• Onboarding never shows the referral code step
• Step flow is Welcome → Skills → Context Gathering (→ done)
• Back/forward navigation works correctly with new step numbering
• ReferralApplyStep.tsx file preserved but not imported in Onboarding.tsx
• No referral API calls during onboarding
• yarn typecheck, yarn lint, yarn format:check, yarn build all pass

Related

• app/src/pages/onboarding/Onboarding.tsx
• app/src/pages/onboarding/steps/ReferralApplyStep.tsx