Better Auth's signIn.social() returns { data: { redirect: true, url } } rather
than issuing an HTTP 30x when using the fetch client. The LoginPage handler
was discarding data.url, so the SSO button appeared to do nothing (the button
disabled but the user never left /login).
Fix: after the social sign-in call, if result.data.url is present, navigate via
window.location.href. Also add an early return in the error branch so the two
paths don't bleed into each other.
Updated UAT_PLAYBOOK.md §5.4.1 TC-WEB-SSO-2 to require a fresh/incognito
context so a stale auth cookie can't mask the regression.
Co-Authored-By: Paperclip <noreply@paperclip.ing>