fix(messages): default message center slug to messages

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

inc/modules/user-parts/urls.php

@@ -60,8 +60,8 @@ function aksrt_get_messages_rewrite_slug(): string {
 	$settings = class_exists('\AKSRT\Settings\Settings')
 		? \AKSRT\Settings\Settings::notification_settings()
 		: [];
-	$slug = sanitize_key((string) ($settings['slug'] ?? 'message'));
-	return $slug !== '' ? $slug : 'message';
+	$slug = sanitize_key((string) ($settings['slug'] ?? 'messages'));
+	return $slug !== '' ? $slug : 'messages';
 }
 endif;