Add GitHub Actions CI for lint, typecheck, and unit tests

[bmendonca3]

Summary

The repository currently has no GitHub Actions workflow, so quality checks rely on manual local runs.

Why this matters

Recent regressions (for example lint failures) can land on main without an automated gate.

Proposed scope

Add a baseline CI workflow that runs on pull_request and push to main:

• npm ci
• npm run lint
• npm run typecheck
• npm test -- --run

Optional follow-up job:

• Playwright smoke/e2e for disconnected flow once deterministic.

Acceptance criteria

• Workflow status appears on PRs.
• PRs clearly show pass/fail for lint/typecheck/unit tests.
• CI config remains fast enough for contributor feedback loops.

Non-goals

• Full release pipeline.
• Heavy matrix expansion in the first iteration.