fix(ci): bump Node.js to v20 in publish-npm workflow

[Copilot]

smob@1.6.1 (transitive dependency) requires Node.js >=20.0.0, causing the publish-npm job to fail when running on Node.js 18.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Changes

• .github/workflows/client-publish.yml — updated node-version from '18' → '20'

Checklist

• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes

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