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/router.ex

@@ -70,7 +70,8 @@ defmodule BerrypodWeb.Router do
         {BerrypodWeb.ThemeHook, :require_site_live},
         {BerrypodWeb.CartHook, :mount_cart},
         {BerrypodWeb.SearchHook, :mount_search},
-        {BerrypodWeb.AnalyticsHook, :track}
+        {BerrypodWeb.AnalyticsHook, :track},
+        {BerrypodWeb.PageEditorHook, :mount_page_editor}
       ] do
       live "/", Shop.Home, :index
       live "/about", Shop.Content, :about