sync editor state with URL for browser back support

- add ?edit=page/theme/settings param when opening editor or switching tabs
- remove ?edit param when closing editor
- restore .themed class on shop_root for editor panel background tokens
- collapse editor when navigating back to URL without ?edit param

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -80,8 +80,9 @@
         This shop works without JavaScript, but some features work better with it enabled.
       </div>
     </noscript>
-    <%!-- Data attributes live on .shop-container inside LiveView for live updates --%>
-    <div class="shop-root">
+    <%!-- .themed here provides fallback theme tokens; .shop-container.themed inside
+         LiveView has data attributes for live updates --%>
+    <div class="shop-root themed">
       {@inner_content}
     </div>
   </body>