chore: upgrade Node.js to 24

[chrisgervang]

Summary

• upgrade the CI and local development runtime from Node.js 18 to Node.js 24.18.0 LTS
• keep the root and website Volta pins aligned with .nvmrc
• remove the Node engine blocker exposed by fix: react-maplibre - support maplibre-gl v6 #2598; that PR still owns its MapLibre-specific lockfile and formatting fixes

Validation

• yarn bootstrap
• yarn lint
• yarn test ci

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Note

Low Risk
Tooling-only version pin changes with no application logic modifications; main risk is environment mismatch for contributors not on Volta/nvm until they upgrade.

Overview
Bumps the repo’s Node runtime from 18 to 24.18.0 LTS by updating .nvmrc and matching Volta pins in the root package.json and website/package.json (18.20.5 → 24.18.0).

GitHub Actions already use node-version-file: '.nvmrc', so CI and local dev pick up the new version from .nvmrc without workflow changes.

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