Document Library settings are missing config schema

Problem/Motivation

Document Library provides settings forms that save configuration to dl.settings.

The saved configuration includes:

- items_per_page
- enable_versioning
- enable_downloads_tracking
- enable_favorites
- enable_comments
- theme
- show_thumbnails

However, the module does not provide a config/schema directory or schema for dl.settings. As a result, the module's configuration cannot be fully validated by Drupal's config schema system.

Steps to reproduce

1. Install Drupal 11.4.
2. Install Document Library.
3. Go to:
/admin/config/content/document-library
or:
/admin/config/content/document-library/settings
4. Change any Document Library setting.
5. Save the form.
6. Run: drush config:inspect --detail --only-error

Proposed resolution

Add config schema for dl.settings.
Suggested schema shape:

dl.settings:
  type: config_object
  label: 'Document Library settings'
  mapping:
    items_per_page:
      type: integer
      label: 'Items per page'
    enable_versioning:
      type: boolean
      label: 'Enable version control'
    enable_downloads_tracking:
      type: boolean
      label: 'Track downloads'
    enable_favorites:
      type: boolean
      label: 'Enable favorites'
    enable_comments:
      type: boolean
      label: 'Enable comments'
    theme:
      type: string
      label: 'Theme'
    show_thumbnails:
      type: boolean
      label: 'Show file thumbnails'