Upgrade frontend and backend runtime to Node 24

[rupertgermann]

Summary

• pin local development and CI to Node 24.18.0 LTS
• require Node 24 for the full-stack Next.js frontend/backend runtime
• align Node type definitions and documentation
• add a contract test preventing runtime-version drift

Validation

• npx tsc --noEmit
• 285 unit tests passed
• 53 sidecar tests passed
• production build passed
• Playwright: 8 passed, 1 hardware-gated test skipped
• clean npm ci completed with 0 vulnerabilities
• scoped ESLint passed for the new test

Baseline notes

• repository-wide npm run lint remains red on 118 pre-existing errors outside this patch
• production build retains an existing Next.js NFT tracing warning

Closes #85