feat: add Footer and OnboardingWizard components - Sprint 3 Complete

Complete Sprint 3 Landing Page development with final 20%:

Footer Component:
- 4-column layout: Brand, Useful Links, Legal, Newsletter
- SVG social icons (GitHub, Twitter, LinkedIn, YouTube)
- Working newsletter form with success feedback
- Dark theme design (slate-900) with proper contrast
- Copyright and attribution footer

OnboardingWizard Component:
- 3-step wizard with progress indicator
- Step 1: Welcome with service explanation
- Step 2: Webhook generation (crypto.randomUUID)
- Step 3: Setup instructions with curl command
- UUID generation and clipboard copy functionality
- Mock implementation (no backend API calls)

Accessibility Features:
- aria-live=polite on wizard container
- aria-current=step for progress indication
- Focus management with useRef/useEffect
- Keyboard navigation support
- Proper ARIA labels on interactive elements

Integration:
- Added to components/index.ts exports
- Integrated in App.tsx after InteractiveDemo
- CTAs scroll to #onboarding section
- Footer placed at page bottom

Build Verification:
- TypeScript compilation: ✓ 0 errors
- CSS bundle: 34KB (gzipped 7KB)
- JS bundle: 238KB (gzipped 72KB)

Sprint 3 Status: 100% Complete ✅
Landing Page now includes:
- Hero, Problem/Solution, HowItWorks, Demo, Onboarding, Footer

Refs: docs/frontend_landing_plan.md

frontend/src/components/index.ts

@@ -1,5 +1,7 @@
 export { Navbar } from './layout/Navbar';
+export { Footer } from './layout/Footer';
 export { Hero } from './sections/Hero';
 export { ProblemSolution } from './sections/ProblemSolution';
 export { HowItWorks } from './sections/HowItWorks';
-export { InteractiveDemo } from './sections/InteractiveDemo';
+export { InteractiveDemo } from './sections/InteractiveDemo';
+export { OnboardingWizard } from './sections/OnboardingWizard';