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>

CLAUDE.md

@@ -38,6 +38,8 @@ src/                  — All plugin PHP (PSR-4 namespace: Unsupervised\Schedula
   AdminMenu.php       — Registers wp-admin menu pages
   RestRegistrar.php   — Registers all REST routes under us-scheduler/v1
   ShortcodeRegistrar.php — Registers [us_booking] and [us_student_login] shortcodes
+  BlockRegistrar.php  — Registers Gutenberg dynamic-block wrappers for the shortcodes
+  BlockPreview.php    — Static editor-preview markup for the blocks
 templates/            — PHP view files included by controllers/shortcodes
 assets/               — CSS and JS (vanilla JS, no build step)
 tests/Unit/           — PHPUnit unit tests (PSR-4: Unsupervised\Schedular\Tests\)
@@ -66,6 +68,8 @@ All database access goes through repository classes within their domain package.
 | `AdminMenu` | Registers wp-admin menu pages |
 | `RestRegistrar` | Registers all REST routes under `us-scheduler/v1` |
 | `ShortcodeRegistrar` | Registers `[us_booking]` and `[us_student_login]` shortcodes |
+| `BlockRegistrar` | Registers Gutenberg dynamic-block wrappers for the shortcodes |
+| `BlockPreview` | Static editor-preview markup for the blocks |
 | `Auth\RoleManager` | Registers `us_instructor` and `us_student` roles with custom caps |
 | `Auth\LoginPage` | Renders front-end student login form |
 | `Availability\AvailabilitySlot` | Immutable value object for a slot row |