fix(sso): add GlotPress compatibility for cross-domain SSO (#749)

GlotPress uses its own gp_head()/gp_footer() template actions instead of
WordPress's wp_head()/wp_footer(). The SSO script is hooked to wp_head, so
it never fires on GlotPress pages, preventing cross-domain login on
translation sites (e.g. translate.example.com).

Register the SSO enqueue_script callback on the gp_head action when
GlotPress is active (GP_VERSION defined). The check is deferred to
plugins_loaded since SSO startup() runs at sunrise before plugins load.

Author: superdav42

inc/sso/class-sso.php

@@ -273,6 +273,22 @@ public function startup(): void {
 
 		add_action('login_head', [$this, 'enqueue_script']);
 
+		/*
+		 * GlotPress uses its own template system with gp_head()/gp_footer()
+		 * instead of wp_head()/wp_footer(). Without this hook the SSO script
+		 * never loads on GlotPress pages, breaking cross-domain login for
+		 * translation sites.
+		 *
+		 * Registration is deferred to plugins_loaded so we can check whether
+		 * GlotPress is actually active before adding the hook.
+		 */
+		add_action('plugins_loaded', function (): void {
+
+			if (defined('GP_VERSION')) {
+				add_action('gp_head', [$this, 'enqueue_script']);
+			}
+		});
+
 		/**
 		 * Allow plugin developers to add additional hooks, if needed.
 		 *