Fix missing "Theme Showcase" sub menu item on Simple Classic site Cal…

…yspo pages (#38698)

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/10242261937

Upstream-Ref: Automattic/jetpack@1d43aff

vendor/automattic/jetpack-mu-wpcom/CHANGELOG.md

@@ -25,6 +25,7 @@ This is an alpha version! The changes listed here are not final.
 - Admin bar: fix icon alignment on small screen widths
 - Adminbar: Fix profile menu colors
 - Admin bar: help center and notification icons now follow color scheme
+- Fix bug so Theme Showcase menu appears on Simple Classic sites
 - Fix RTL admin bar
 - Masterbar: Fix icon overlap issue at smaller resolutions
 - MU WPCOM: Fix the option of the MailerLite widget is gone

vendor/automattic/jetpack-mu-wpcom/src/features/wpcom-themes/wpcom-themes.php

@@ -9,14 +9,14 @@
 
 use Automattic\Jetpack\Jetpack_Mu_Wpcom;
 
-if ( get_option( 'wpcom_admin_interface' ) !== 'wp-admin' ) {
-	return;
-}
-
 /**
  * Displays a banner before the theme browser that links to the WP.com Theme Showcase.
  */
 function wpcom_themes_show_banner() {
+	if ( get_option( 'wpcom_admin_interface' ) !== 'wp-admin' ) {
+		return;
+	}
+
 	$site_slug        = wp_parse_url( home_url(), PHP_URL_HOST );
 	$wpcom_logo       = plugins_url( 'images/wpcom-logo.svg', __FILE__ );
 	$background_image = plugins_url( 'images/banner-background.webp', __FILE__ );
@@ -56,6 +56,10 @@ function wpcom_themes_show_banner() {
  * Registers an "Appearance > Theme Showcase" menu.
  */
 function wpcom_themes_add_theme_showcase_menu() {
+	if ( get_option( 'wpcom_admin_interface' ) !== 'wp-admin' ) {
+		return;
+	}
+
 	$site_slug = wp_parse_url( home_url(), PHP_URL_HOST );
 	add_submenu_page(
 		'themes.php',