ci: add Playwright smoke job to GitHub Actions

[ugoocreates-pixel]

Closes #437

Resolves the issue to add the Playwright smoke job into GitHub Actions CI scope.

• Added a new smoke-test job in .github/workflows/ci.yml that caches Playwright browsers sensibly using actions/cache with the pnpm-lock.yaml hash.
• Documented the pnpm test:e2e requirement in CONTRIBUTING.md for local testing.

Acceptance Criteria

• CI runs smoke on PRs
• Documented in CONTRIBUTING
• Non-blocking or required smoke job (implemented as a parallel, non-blocking job by default)
• Cache browsers sensibly

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