Upgrade Next to 16.3.8 and default the store domain to mock.shop

An unset or blank NEXT_PUBLIC_SHOPIFY_DOMAIN reached the Hydrogen client
as an empty storeDomain, which throws 'storeDomain is required and must
be non-empty'. Fall back to the tokenless mock.shop store, only send the
public token alongside a configured domain, and have the editor's Shopify
plugin read the same resolved config.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/shopify/client.ts

@@ -30,7 +30,7 @@ const fetchWith = (overrides: RequestInit): typeof globalThis.fetch =>
   }) as typeof globalThis.fetch;
 
 const config = {
-  storeDomain: SHOPIFY_STORE_DOMAIN!,
+  storeDomain: SHOPIFY_STORE_DOMAIN,
   apiVersion: SHOPIFY_API_VERSION,
   publicStorefrontToken: SHOPIFY_PUBLIC_ACCESS_TOKEN,
 };