Align Node.js CI matrix with project runtime requirements

[Copilot]

build (18.x) was failing in Actions because the current Vite/Rolldown toolchain requires Node 20+, while the workflow still included Node 18. This PR updates CI to run only on supported Node versions and avoids false-negative test step failures when no test script is defined.

• CI runtime compatibility

  • Remove 18.x from the Node matrix in .github/workflows/node.js.yml
  • Keep CI coverage on supported LTS versions (20.x, 22.x)

• Test step resilience

  • Change npm test to npm test --if-present so repositories without a test script do not fail the pipeline by default

strategy:
  matrix:
    node-version: [20.x, 22.x]

steps:
  - run: npm ci
  - run: npm run build --if-present
  - run: npm test --if-present