Support OIDC check session iframe

`check_session_iframe` is defined in [OIDC Session Management](https://openid.net/specs/openid-connect-session-1_0.html). With check session iframe, web application would be able to be notified when user logout via `end_session_endpoint`

Start with the linked OIDC Session Management specification, focusing on check_session_iframe and end_session_endpoint. The issue names no repository files, tests, or entry points, so trace the existing logout and session-management flow first. Done means implementing the specified iframe support and logout notification behavior.

Written by the indexing model from the issue text.