CI: Add Vercel preview smoke test to frontend PR workflow

[talissoncosta]

Context

We don't currently have automated verification that Vercel preview deployments are healthy after a frontend PR push. Build failures, broken Express server configs, or misconfigured environment variables can ship to preview without anyone noticing until a reviewer manually opens the link.

Proposal

Add a lightweight smoke test to the frontend PR workflow that checks Vercel preview deployments after each push. The test should:

• Verify the Express server is responding (/config/project-overrides)
• Verify static files are served (/)
• Be non-blocking (warn but don't gate merge)
• Handle Vercel's "Ignored Build Step" (no frontend changes → previous build still live)

Implementation

PR #7192 implements this by parsing preview URLs from the Vercel bot PR comment and curling key endpoints.

Notes

• The test is intentionally non-blocking (continue-on-error: true) to avoid false positives blocking PRs

[talissoncosta]

Closing — dropping this. The implementation relied on parsing the Vercel bot's PR comment to find the preview URL, which is too fragile a signal to build CI on top of. If we revisit this, the preview URL should come from Vercel's own deployment status or API rather than a scraped comment.