fix: make deploy validation reliable

[kentcdodds]

Summary

• install only Playwright’s Chromium headless shell and Linux dependencies before solution tests
• run browser-backed tests on Node 24 LTS because Playwright’s installer hangs while extracting under Node 26; cross-platform typecheck/lint coverage remains on Node 26
• replace unbounded ESLint worker concurrency with a single process and a 6 GB heap, below hosted-runner memory limits

Validation

• npm run lint
• npm run typecheck
• npx playwright install --with-deps --only-shell chromium
• node ./epicshop/test.js ..s (all 10 solution suites pass)
• CI setup/lint passes on Ubuntu, Windows, and macOS; Node 24 browser-install retry pending

Fixes the failures in https://github.com/epicweb-dev/react-performance/actions/runs/30414173241