Add merge-queue cross-browser preview smoke tests

[neonwatty]

Summary

• add focused cross-browser live Playwright projects for Chromium, Firefox, and WebKit
• add live preview smoke coverage for widget asset loading, no-screenshot submit, and complex screenshot options
• run the smoke suite in a merge-queue-only browser matrix after preview deploy
• add a local Makefile helper and document the implementation plan

Verification

• npm run format:check
• npm run typecheck
• npm run lint (existing warnings only)
• npm test
• npx playwright test --list --project=chromium-cross-browser-live
• npx playwright test --list --project=firefox-cross-browser-live
• npx playwright test --list --project=webkit-cross-browser-live
• npx playwright test --list --project=chromium | rg "cross-browser-live|widget.cross" || true

Notes

• Local live smoke execution against the Vercel venue is blocked without VERCEL_AUTOMATION_BYPASS_SECRET; merge-queue CI passes that secret into the test job.

[github-actions]

🎉 This PR is included in version 1.33.3 🎉

The release is available on GitHub release

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