Add Playwright smoke tests to CI workflow

[Copilot]

Pull Request Template: Add a Conference or Workshop

Thank you for contributing! Most PRs are to add a new conference or workshop. The following are to help ensure you've added everything correctly:

Conference/Workshop Details

• Name, Location, Date(s), Website URL and Status are required
• X / Twitter is Optional
• Location should be City + Country or Online
• Status can include information about registration, pricing, calls for proposals and can include links to all the above.

Checklist

• I have added the conference/workshop to the correct YAML file (_data/current.yml or _data/past.yml)
• The entry includes name, location, date(s), url and accurate status
• The entry is in chronological order (soonest to furtherest away)
• If there are any special characters in the name field (: or ; or '), the name must be in quotes (")
• I have checked for duplicates to avoid listing the same event twice
• Build runs successfully

Additional context

This PR adds a minimal Playwright test harness and runs it in GitHub Actions CI to provide lightweight browser automation coverage for key site paths. The suite includes both API-style response checks and UI smoke checks.

• Playwright project bootstrap

  • Added package.json / package-lock.json with @playwright/test
  • Added playwright.config.js to run tests from test/playwright against built _site via local static server

• Smoke tests (API vs UI)

  • Added test/playwright/site-smoke.spec.js
  • API-style: verifies successful responses for /, /past/, /subscribe/
  • UI-style: verifies homepage title/nav links and that /past/ renders conference list rows

• CI integration

  • Updated .github/workflows/ci.yml to install Node deps, install Chromium, run Playwright tests, and upload Playwright report artifact
  • Updated .gitignore for Playwright/node artifacts

test('API: key pages return success responses', async ({ request, baseURL }) => {
  for (const path of ['/', '/past/', '/subscribe/']) {
    const response = await request.get(`${baseURL}${path}`);
    expect(response.ok()).toBeTruthy();
  }
});

• Fixes Build out some automated tests as part of the CI process #488