Add UAT_PLAYBOOK.md covering VITE_API_URL auth resolution:
- TC-AUTH-4.1.x: Tests for when VITE_API_URL is set
- TC-AUTH-4.2.x: Tests for when VITE_API_URL is unset (window.location.origin fallback)
- TC-AUTH-4.3.x: Session persistence tests
Updated UAT_PLAYBOOK.md §4 — auth base URL resolution test cases.
GRO-1289