FIX Stabilize Playwright frontend host resolution

[romanlutz]

Description

Playwright used localhost:3000 while its Vite server and readiness checks used 127.0.0.1. On Windows, localhost can resolve to IPv6 ::1, causing intermittent ECONNREFUSED failures because Vite is listening on IPv4.

Define one IPv4 frontend URL and reuse it for Playwright's base URL, storage-state origin, and web-server readiness checks. This also keeps the onboarding local-storage state scoped to the same origin used by tests.

Tests and Documentation

• npm run type-check
• Affected Response flows repeated 3 times with 8 workers and retries disabled: 12 passed
• Complete 123-test mock and seeded suite run twice with 8 workers: no ECONNREFUSED, ::1, or localhost:3000 requests observed
• Existing unrelated concurrency flakes remained in history loading, backend setup, and video rendering
• Documentation not applicable