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>

package.json

@@ -13,7 +13,7 @@
     "@ai-sdk/react": "^4.0.47",
     "@chakra-ui/react": "^3.35.0",
     "@emotion/react": "^11.14.0",
-    "@next/swc-wasm-web": "16.2.10",
+    "@next/swc-wasm-web": "16.3.8",
     "@openrouter/ai-sdk-provider": "^3.0.0",
     "@radix-ui/react-slot": "^1.3.3",
     "@radix-ui/react-use-controllable-state": "^1.2.6",
@@ -34,7 +34,7 @@
     "lucide-react": "^0.562.0",
     "motion": "^12.23.26",
     "nanoid": "^6.0.0",
-    "next": "16.2.10",
+    "next": "16.3.8",
     "radix-ui": "^1.6.7",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",