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]>

tests/Unit/BlockPreviewTest.php

@@ -29,6 +29,17 @@ class BlockPreviewTest extends TestCase
         self::assertStringContainsString('class="us-class"', $html);
         self::assertStringContainsString('class="us-enrol-btn" disabled', $html);
         self::assertStringContainsString('us-editor-note', $html);
+        self::assertStringContainsString('A sample class shown so the page can be styled.', $html);
+    }
+
+    public function testSingleClassGroupPreviewDropsTheDescriptionButKeepsScheduleAndEnrolment(): void
+    {
+        $html = BlockPreview::groupClasses(true);
+
+        self::assertStringNotContainsString('A sample class shown so the page can be styled.', $html);
+        self::assertStringContainsString('class="us-class-when"', $html);
+        self::assertStringContainsString('class="us-enrol-deadline"', $html);
+        self::assertStringContainsString('class="us-enrol-btn" disabled', $html);
     }
 
     public function testLoginPreviewIncludesTheRealLoginTemplate(): void