fix: bump next to 16.2.4 in frontend

From the PR description

Summary

Fixes the broken npm install --prefix frontend reported during local setup.

The frontend pinned next@16.0.3, but @opennextjs/cloudflare@^1.13.1 resolves to 1.19.6, which requires next peer >=15.5.15 <16 || >=16.2.3. This caused ERESOLVE errors and forced contributors to use --legacy-peer-deps. Additionally, next@16.0.3 is flagged by CVE-2025-66478.

Changes

• frontend/package.json
  • next: 16.0.3 → 16.2.4
  • eslint-config-next: 16.0.3 → 16.2.4
• frontend/package-lock.json regenerated

Verification

• rm -rf node_modules package-lock.json && npm install completes cleanly with no --legacy-peer-deps.
• npm run build --prefix frontend compiles successfully and TypeScript passes (the prerender step requires populated frontend/.env.local, which is expected for fresh local setups).
• Vulnerabilities reported by npm audit: 9 (1 critical, 1 high, 7 moderate) → 8 (all moderate).

Test plan

• Pull branch on a clean checkout, run npm install --prefix frontend without flags - expect success.
• Populate frontend/.env.local and run npm run build --prefix frontend - expect end-to-end build success.
• Run npm run dev --prefix frontend and smoke-test the app at http://localhost:3000.