"custom_system_prompt" not working correctly

The "custom_system_prompt" can be entered and saves correctly.
But when displaying the settings form, the wrong variable is used, causing a new save on the form to clear the "custom_system_prompt" again.

A small code change is needed in "\Drupal\ai_seo\Form\ConfigurationForm line 111":

      '#default_value' => $config->get('system_prompt') ?? '',

Needs to be change to:

      '#default_value' => $config->get('custom_system_prompt') ?? '',