ci: run pull request tests in GitHub Actions

[jevin98]

Summary

• Add a pull request GitHub Actions workflow for linting, type-checking, unit tests, and e2e tests.
• Install Playwright Chromium in CI before running e2e tests.
• Remove the local Chrome/Edge executable fallback from Playwright config so e2e requires Playwright-managed browsers.

Validation

• pnpm exec eslint . --fix
• pnpm exec eslint .
• pnpm compile
• pnpm test:unit

Notes

• Local pnpm test:e2e built the Chrome extension, then failed because Playwright Chromium was not installed.
• Local pnpm exec playwright install chromium exceeded the 5-minute desktop timeout. The CI workflow now runs pnpm exec playwright install --with-deps chromium before e2e tests.