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>

docs/features/editor-blocks.md

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+# Editor Blocks
+
+Gutenberg dynamic-block wrappers for the plugin's four front-end shortcodes,
+so the pages can be previewed and styled inside the block editor instead of
+appearing as grey shortcode text.
+
+## Blocks
+
+| Block | Wraps shortcode | Front-end renderer |
+|---|---|---|
+| `us-scheduler/booking` | `[us_booking]` | `Booking\BookingPage::render()` |
+| `us-scheduler/student-login` | `[us_student_login]` | `Auth\LoginPage::render()` |
+| `us-scheduler/student-register` | `[us_student_register]` | `Auth\RegistrationPage::render()` |
+| `us-scheduler/group-classes` | `[us_group_classes]` | `GroupClass\GroupClassPage::render()` |
+
+The shortcodes remain registered for back-compat; blocks and shortcodes share
+the same page objects (constructed once in `Plugin::boot()`), so front-end
+output is identical either way. Pasting a shortcode into the block editor
+auto-converts it to the matching block via a `transforms.from` shortcode
+transform.
+
+## How it works
+
+- **`BlockRegistrar`** (`src/BlockRegistrar.php`) hooks `init` and registers
+  each block with `register_block_type()`: a `render_callback` per block, the
+  shared editor script (`assets/js/blocks.js`, handle
+  `us-scheduler-blocks`), and the front-end stylesheet
+  (`assets/css/frontend.css`, handle `us-scheduler`) as the block `style` so
+  it also loads inside the editor and previews pick up theme styling.
+- **`assets/js/blocks.js`** (vanilla JS, no build step) registers the client
+  side of each block — title, icon, keywords, shortcode transform — and
+  renders the editor preview with `wp.serverSideRender`, which fetches the
+  server-rendered markup via the `/wp/v2/block-renderer` REST route.
+- **`BlockPreview`** (`src/BlockPreview.php`) supplies static, script-free
+  markup for editor previews. `BlockRegistrar::isEditorPreview()` detects the
+  block-renderer context via the `REST_REQUEST` constant (front-end template
+  rendering never happens inside a REST request) and renders the preview
+  instead of the live page.
+
+## Editor preview behaviour
+
+Live pages cannot run in the editor: booking and group classes are populated
+by JavaScript making authenticated REST calls (and may load Stripe.js),
+registration requires a valid invite token, and login short-circuits for
+logged-in users (the editing admin always is). Each preview therefore
+reproduces the live wrapper elements and CSS classes with representative
+placeholder content:
+
+- **Booking** — `#us-booking-app` with sample `.us-day` / `.us-slot` rows and
+  disabled Book buttons.
+- **Group classes** — `#us-group-app` with a sample `.us-class` card and a
+  disabled Enrol button.
+- **Login** — the real `templates/frontend/login-page.php` template (it has
+  no request-state dependencies).
+- **Registration** — a disabled sample of the `.us-register-form` fields.
+
+Each preview starts with a `.us-editor-note` paragraph explaining what the
+published page shows instead. The note class only appears in editor previews.
+
+## Tests
+
+- `tests/Unit/BlockRegistrarTest.php` — hook registration, block/asset
+  registration, front-end delegation to the page objects, preview-mode
+  routing.
+- `tests/Unit/BlockPreviewTest.php` — preview markup mirrors the live CSS
+  classes/ids and includes the editor note.