Omit the class description when the group block shows one class

The Group Classes block can be pinned to a single class via its Class
option so it can be embedded on a page dedicated to that class. On such a
page the surrounding copy already describes the class, so the card
repeated it. In single-class mode the description is now left out and the
card shows only the schedule, instructor, schedule note, price, enrolment
deadline and the enrol/withdraw controls.

The editor preview follows the same rule: BlockPreview::groupClasses()
takes the mode from the block's offeringId attribute, drops the sample
description when a class is pinned, and notes what the published page
shows. Its sample card also gained the .us-class-when and
.us-enrol-deadline elements the live markup has always rendered.

Closes #114

Co-Authored-By: Claude Opus 5 <[email protected]>

src/BlockRegistrar.php

@@ -160,7 +160,11 @@ class BlockRegistrar {
 	 * @param array<string, mixed> $attributes Block attributes.
 	 */
 	public function renderGroupClasses( array $attributes = [] ): string {
-		return $this->isEditorPreview() ? BlockPreview::groupClasses() : $this->groupClassPage->render( $attributes );
+		if ( ! $this->isEditorPreview() ) {
+			return $this->groupClassPage->render( $attributes );
+		}
+
+		return BlockPreview::groupClasses( Val::int( $attributes['offeringId'] ?? 0 ) > 0 );
 	}
 
 	/**