add live page editor sidebar with collapsible UI

Admins can now edit pages directly on the live shop by clicking the
pencil icon in the header. A sidebar slides in with block management
controls (add, remove, reorder, edit settings, save, reset, done).

Key features:
- PageEditorHook on_mount with handle_params/event/info hooks
- BlockEditor pure functions extracted from admin editor
- Shared BlockEditorComponents with event_prefix namespacing
- Collapsible sidebar: X closes it, header pencil reopens it
- Backdrop overlay dismisses sidebar on tap
- Conditional admin.css loading for logged-in users
- content_body block now portable (textarea setting + rich text fallback)

13 integration tests, 26 unit tests, 1370 total passing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod_web/components/layouts/shop_root.html.heex

@@ -63,6 +63,9 @@
       @layer properties, reset, primitives, tokens, theme, base, components, layout, utilities, overrides;
     </style>
     <link phx-track-static rel="stylesheet" href={~p"/assets/css/shop.css"} />
+    <%= if assigns[:current_scope] && @current_scope.user do %>
+      <link phx-track-static rel="stylesheet" href={~p"/assets/css/admin.css"} />
+    <% end %>
     <script defer phx-track-static src={~p"/assets/js/app.js"}>
     </script>
     <!-- Generated theme CSS with @font-face declarations -->