fix(embedded-e2e): force IPv4 base URL for Playwright CI

`flask run` binds IPv4 only, but Node's DNS resolution for `localhost`
on the GH runner can return `::1` (IPv6) first, surfacing as
`connect ECONNREFUSED ::1:8081` once happy-eyeballs flips after the
first read-reset. The embedded suite is uniquely exposed because it
drives heavy Node-side API calls (CSRF + guest token) where the
regular Playwright suite is browser-driven.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>