Remove quantityAvailable from product variant query

The Storefront API only returns ProductVariant.quantityAvailable when the
app has the unauthenticated_read_product_inventory scope, and nothing in
the template reads the field — availableForSale already drives stock state.

Also hides the editor left sidebar by default in PageEditor.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FaxyZTjAyd7UXK8xpQpF7o

components/page-editor.tsx

@@ -42,6 +42,7 @@ export default function PageEditor({ pagePath, page }: PageEditorProps) {
     <Editor
       theme="light"
       color="teal"
+      ui={{ leftSideBarVisible: false }}
       config={appConfig as any}
       data={data}
       plugins={plugins}