Issue #3416991: Include correct CKEditor 5 stylesheets for active domain theme

Problem

When using a different theme per domain, CKEditor 5 does not load the correct stylesheets (ckeditor5-stylesheets from the theme's .info.yml). This happens because the CKEditor 5 module uses the default system theme instead of the active domain theme.

Solution

Implements hook_library_info_alter() to detect the active domain via domain.negotiator, look up its configured default theme, and inject that theme's CKEditor 5 stylesheets into the internal.drupal.ckeditor5.stylesheets library.

Changes

• domain_theme_switch.module: adds domain_theme_switch_library_info_alter()

Fixes #3416991