fix(messages): default message center slug to messages

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

src/Settings/Integration.php

@@ -209,9 +209,9 @@ class Integration {
      */
     public static function notification_settings(): array {
         $settings = Settings::get_all();
-        $slug = sanitize_key((string) ($settings['msg_center_rewrite_slug'] ?? 'message'));
+        $slug = sanitize_key((string) ($settings['msg_center_rewrite_slug'] ?? 'messages'));
         if ($slug === '') {
-            $slug = 'message';
+            $slug = 'messages';
         }
 
         $paginate = sanitize_key((string) ($settings['message_paginate_type'] ?? 'ajax_lists'));