This page is the login_uri that google-popup-login.html
declares to Google Identity Services (GSI). GSI requires this attribute to exist even when the primary
sign-in flow runs entirely through the JS callback in popup mode.
This is a static test site with no server, so it cannot read the POST body Google sends here
(credential + g_csrf_token form fields) — that requires a backend endpoint.
A production integration would read those two fields here, verify g_csrf_token matches the
g_csrf_token cookie, verify the credential JWT, create a session, then redirect the user in.