Add CI workflow: lint + format + typecheck

[nora-weisser]

Goal

Add a CI workflow that gives fast, deterministic feedback on every PR without needing a backend or credentials.

Scope

Create .github/workflows/ci.yml:

• Triggers: pull_request and push to the default branch.
• Steps: checkout → setup-node (v20, npm cache) → npm ci → npm run lint → npm run format (check mode) → npm run typecheck.
• Do not run Playwright tests here (no backend / creds in CI).

Full E2E-against-a-backend is tracked separately (see the deferred E2E issue).

Acceptance criteria

• .github/workflows/ci.yml runs on PR and default-branch push
• lint, format-check, and typecheck all run and gate the PR
• Validated by opening a draft PR and confirming checks run green