chore: align tooling with Node 20

[WA11AX]

Summary

• enforce Node.js 20 across project and CI
• migrate ESLint to flat config with strict warnings
• configure Renovate for esbuild, vite and tsx updates

Testing

• npm pkg get overrides.esbuild
• npm ls esbuild
• node -e "console.log(require('esbuild').version)"
• npm audit --omit=dev --audit-level=high
• npm run lint
• npm test

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https://chatgpt.com/codex/tasks/task_e_689c4f8ec3708327b823bd5deb511f9f

[WA11AX]

Run npm run format:check

rest-express@1.0.0 format:check
prettier --check .
Checking formatting...
[warn] eslint.config.js
[warn] package.json
[warn] Code style issues found in 2 files. Run Prettier with --write to fix.
Error: Process completed with exit code 1.