Site polish: footer, copy button, OG image, 404, example metadata

[raj-khan]

Summary

• Footer — replaced all href="#" dead links with real GitHub URLs (SECURITY.md, LICENSE, CONTRIBUTING.md); fixed "Docs" link to /docs; updated brand tagline to OSS messaging
• CodeBlock copy button — wired navigator.clipboard.writeText with 1.8s "copied!" feedback state
• OG image — updated headline, description, and footer tags to match OSS/open-source positioning
• Custom 404 — not-found.tsx with branded design (Nav, serif headline, 3 CTA buttons, Footer)
• Example page metadata — added layout.tsx in /example to carry title + description (page itself stays 'use client')

Test plan

• Visit / — footer links resolve (no href="#" dead ends); "Docs" goes to /docs
• Developers section — copy button shows "copied!" briefly then reverts
• /404-nonexistent-path — branded 404 renders with all three CTA buttons
• Browser tab on /example shows "Live example · LearnKit AI"
• OG image at /opengraph-image renders updated headline
• pnpm --filter @learnkit-ai/web typecheck passes