Convert WPML integration to WPML String Package

https://wpml.org/documentation/support/string-package-translation/

if ( ! function_exists( 'cptui_pkg_build_package' ) ) {

    function cptui_pkg_build_package( $slug ) {
        return array(
            'kind'      => 'CPT UI Post Type',
            'kind_slug' => 'cptui-post-type',
            'name'      => $slug,
            'title'     => 'Post Type: ' . $slug,
            'edit_link' => admin_url( 'admin.php?page=cptui_manage_post_types&action=edit' ),
        );
    }

    add_filter( 'wpml_active_string_package_kinds', function ( $kinds ) {
        $kinds['cptui-post-type'] = 'CPT UI Post Type';
        return $kinds;
    } );

    // Register the package + its label strings (here scoped to 'book').
    add_action( 'wp_loaded', function () {
        if ( ! defined( 'WPML_ST_VERSION' ) ) { return; }
        $all = get_option( 'cptui_post_types', array() );
        if ( empty( $all['book'] ) ) { return; }
        $pt      = $all['book'];
        $package = cptui_pkg_build_package( 'book' );
        do_action( 'wpml_start_string_package_registration', $package );
        if ( ! empty( $pt['label'] ) ) {
            do_action( 'wpml_register_string', $pt['label'], 'name', $package, 'Name (plural)', 'LINE' );
        }
        if ( ! empty( $pt['singular_label'] ) ) {
            do_action( 'wpml_register_string', $pt['singular_label'], 'singular_name', $package, 'Singular name', 'LINE' );
        }
        if ( ! empty( $pt['labels'] ) && is_array( $pt['labels'] ) ) {
            foreach ( $pt['labels'] as $key => $val ) {
                if ( '' === $val ) { continue; }
                do_action( 'wpml_register_string', $val, $key, $package, $key, 'LINE' );
            }
        }
        do_action( 'wpml_delete_unused_package_strings', $package );
    } );

    // Apply the package translations when CPT UI registers the post type.
    add_filter( 'cptui_pre_register_post_type', function ( $args, $slug, $post_type ) {
        if ( 'book' !== $slug || ! defined( 'WPML_ST_VERSION' ) ) { return $args; }
        $package = cptui_pkg_build_package( 'book' );
        if ( ! empty( $args['labels'] ) && is_array( $args['labels'] ) ) {
            foreach ( $args['labels'] as $key => $val ) {
                $args['labels'][ $key ] = apply_filters( 'wpml_translate_string', $val, $key, $package );
            }
        }
        if ( ! empty( $args['label'] ) ) {
            $args['label'] = apply_filters( 'wpml_translate_string', $args['label'], 'name', $package );
        }
        return $args;
    }, 20, 3 );
}

Start with the WPML String Package documentation linked in the issue, then trace the existing CPT UI WPML integration around cptui_pre_register_post_type and the wp_loaded registration flow. Generalize the demonstrated package registration and translation handling beyond the example book post type, and verify that custom post type labels are registered, translated, and unused package strings are removed.