feat: implement public credential verification portal (Issue #376)

[Peolite001]

Description

Closes #376

This PR introduces a public, login-free portal where anyone can verify an issued credential by its ID/hash. It provides an external-facing surface for employers, institutions, and learners to prove a credential is genuine.

Changes Made

• Smart Contract (contracts/src/credential_registry.rs & contracts/src/lib.rs): Added a get_credential_read_only helper to allow queries without triggering expiration state mutations.
• Backend Route (backend/src/routes/verify.ts): Implemented a new public /api/v1/verify/:hash endpoint that resolves credentials via the Soroban contract and properly surfaces error states (e.g., Revoked, Expired).
• Backend Tests (backend/src/tests/verify.test.ts): Added comprehensive E2E tests for the verification endpoint to validate behavior against valid, revoked, and unknown credentials.
• Frontend Portal (frontend/src/app/verify/[hash]/page.tsx): Built a clean, responsive page to display credential details.
• Frontend Components (frontend/src/components/verify/):
  • CredentialDisplay.tsx: Renders credential metadata and dynamically displays a prominent status badge.
  • QRCodeDisplay.tsx: Generates a shareable QR code pointing to the specific credential portal link.

Acceptance Criteria Met

• A public /verify/:hash page renders credential details without authentication
• QR code and deep-link sharing for every credential
• A public API endpoint returns the canonical credential state from the contract
• Revoked/invalid credentials render an explicit "invalid" state with reason
• Portal is indexable, responsive, and passes the accessibility audit
• End-to-end tests cover valid, revoked, and unknown hashes

How to Test

1. Start the local backend and frontend servers.
2. Ensure you have the AETHERMINT_CONTRACT_ID available in your backend env.
3. Run the backend tests: npm test verify.test.ts
4. Navigate to http://localhost:3000/verify/1 (replace 1 with a valid mock or real credential ID) to view the portal.
5. Ensure the QR code scans correctly and points to the live URL.