Default the store domain to mock.shop when unset

An unset or blank VITE_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 instead, and only
send the public token alongside a configured domain.

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

src/services/shopify/client.ts

@@ -58,7 +58,7 @@ export const storefront = createStorefrontClient({
   type: 'public',
   requestContext,
   config: {
-    storeDomain: SHOPIFY_STORE_DOMAIN!,
+    storeDomain: SHOPIFY_STORE_DOMAIN,
     apiVersion: SHOPIFY_API_VERSION,
     publicStorefrontToken: SHOPIFY_PUBLIC_ACCESS_TOKEN,
     fetch: storefrontFetch,

src/services/shopify/config.ts

@@ -2,15 +2,25 @@
 //
 // These are all `VITE_*`, so Vite inlines them at build time and they are safe
 // to read from the browser bundle as well as the Hono server.
-export const SHOPIFY_STORE_DOMAIN = import.meta.env.VITE_SHOPIFY_DOMAIN;
+const CONFIGURED_STORE_DOMAIN = import.meta.env.VITE_SHOPIFY_DOMAIN?.trim();
+
+/**
+ * Store domain, falling back to the tokenless `mock.shop` demo store when the
+ * variable is unset or blank — the Storefront client rejects an empty domain.
+ */
+export const SHOPIFY_STORE_DOMAIN = CONFIGURED_STORE_DOMAIN || 'mock.shop';
 
 /**
  * Public Storefront API access token. Safe to expose to the browser — that is
  * what "public" means here. Omitted for tokenless storefronts such as
  * `mock.shop`. Never put a *private* token behind a `VITE_` name.
+ *
+ * Only used alongside a configured domain: a token belongs to one store, so it
+ * is never sent to the `mock.shop` fallback.
  */
-export const SHOPIFY_PUBLIC_ACCESS_TOKEN = import.meta.env
-  .VITE_SHOPIFY_PUBLIC_ACCESS_TOKEN;
+export const SHOPIFY_PUBLIC_ACCESS_TOKEN = CONFIGURED_STORE_DOMAIN
+  ? import.meta.env.VITE_SHOPIFY_PUBLIC_ACCESS_TOKEN?.trim() || undefined
+  : undefined;
 
 export const SHOPIFY_API_VERSION =
   import.meta.env.VITE_SHOPIFY_API_VERSION || '2026-07';