Set up GitHub Actions CI

[nora-weisser]

Description:

Add a CI pipeline that runs on every push and pull request to main, so regressions and style/type issues are caught automatically. The workflow installs dependencies, type-checks, lints, and runs the Playwright suite, uploading the HTML report as an artifact.

Tasks

• Add .github/workflows/ci.yml triggered on push and pull_request to main.
• Cache npm dependencies and install Playwright browsers.
• Run steps: typecheck, lint, format:check, test.
• Provide secrets/credentials via GitHub Actions secrets (no committed .env).
• Upload the Playwright HTML report (and traces on failure) as workflow artifacts.
• Add a status badge to README.md.

Acceptance criteria

• CI runs automatically on PRs to main and blocks merge on failure.
• Secrets come from GitHub Actions secrets, never the repo.
• HTML report/traces are available as downloadable artifacts.