feat: shareable credential URLs + domain certificates

- Add #/verify/HASH/BASE64 route: credential data encoded in URL so
  third parties can view the full proof card without localStorage access
- Add Share Proof button to results screen (copies link to clipboard)
- Add renderVerify() — public-facing credential card with grade, score,
  mode, integrity, time, issued date, and SHA-256 tamper-evident proof
- Add checkDomainCertificate(): awards cert when all challenges in a
  domain are completed at grade B (≥70%) or better
- Domain certs persisted to localStorage, displayed on dashboard
- Toast fires on domain cert unlock

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: RJ Lopez