Set the just-logged-out flag before navigating, and skip the intermediate
router.push to login when redirecting to the IdP — otherwise Login.vue's
onBeforeMount consumed the flag before the logout round-trip landed, so the
single-provider auto-redirect fired and logged the user straight back in.
redirectToProviderOnLogout now reports whether it navigated, so logout can fall through to the login page when there's no static logout URL.