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

graphql/products.ts

@@ -48,7 +48,6 @@ export const ProductFragment = gql(`
           id
           title
           availableForSale
-          quantityAvailable
           selectedOptions {
             name
             value