feat: build waitlist analytics & A/B testing infrastructure (#68)

[devJaja]

Summary

Closes #68

Implements the full waitlist analytics and A/B testing infrastructure as specified.

Changes

AnalyticsProvider (app/(landing)/waitlist/components/AnalyticsProvider.tsx)

• React context provider exposing track(event, properties), giveConsent, revokeConsent, consentGiven
• Tracks all 6 required events: page_view, cta_click, tier_selection, payment_attempt, successful_signup, referral_share
• Forwards to GA4 (gtag) and Mixpanel via thin adapters — swap in real SDK calls without touching business logic
• Events fired before consent are queued and flushed automatically on acceptance
• PII redaction: any property key matching email|name|phone|address|ip is replaced with [redacted]
• GDPR/CCPA consent banner rendered at page bottom; GA4 script injected only after consent (anonymize_ip: true)
• Consent persisted in a 1-year cookie; revocation clears the queue

useABTest (src/hooks/useABTest.ts)

• Custom A/B testing hook — no external SDK required; experiments start/stop without code deployment
• Deterministic assignment: hash of experimentKey + stableUserId → consistent bucketing per user
• Supports weighted distributions (e.g. 90/10 split via weights option)
• Assignments persisted in localStorage; stable across page loads
• isLoading: true during SSR/first render, resolves on mount (no hydration flash)

Waitlist landing page (app/(landing)/waitlist/)

• page.tsx: wraps content in AnalyticsProvider
• WaitlistContent.tsx: runs hero_headline 3-variant A/B test, fires page_view with variant attached

Tests

• 10 tests for AnalyticsProvider: banner visibility, consent state, cookie persistence, event tracking, accessibility (axe)
• 8 tests for useABTest: variant assignment, stability, localStorage persistence, invalid-value recovery, weighted distribution, multi-experiment independence

What was tested

• All CI checks: npm run lint + npm run test:coverage (new tests are in the jsdom project, coverage threshold unaffected as new files are not in collectCoverageFrom globs)