Update required Node.js version

[crandmck]

The site actually requires Node >=24.15.0.

If you do npm install using Node v24.13.0, you get this error:

npm error EBADDEVENGINES   current: { name: 'node', version: 'v24.13.0' },
npm error EBADDEVENGINES   required: { name: 'node', version: '>=24.15.0', onFail: 'error' }

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[krzysdz]

The version was updated to 24.15 in package.json and .nvmrc in #2320.