Add Gutenberg dynamic-block wrappers for the front-end shortcodes

Wrap the four shortcodes (us_booking, us_student_login,
us_student_register, us_group_classes) in dynamic blocks so pages can be
previewed and styled in the block editor. Front-end rendering delegates
to the same page objects the shortcodes use; in the editor's
block-renderer REST preview a static, script-free BlockPreview is
rendered instead (no live REST calls, redirects, or Stripe.js). The
editor script (vanilla JS, no build step) registers each block with
wp.serverSideRender previews and shortcode transforms; frontend.css is
attached as the block style so previews pick up theme styling.

Resolves #44

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/css/frontend.css

@@ -33,3 +33,10 @@
     color: #c00;
     margin-top: 8px;
 }
+
+/* Shown only in block-editor previews (see BlockPreview). */
+.us-editor-note {
+    font-size: 0.85em;
+    font-style: italic;
+    opacity: 0.7;
+}