feat(studio): deploy manual designer and configurable AI model

.env.example

@@ -1,4 +1,6 @@
 # Server only. Never put this key in browser scripts or commit your .env file.
 APIMART_API_KEY=
 STUDIO_PUBLIC_ORIGIN=https://component.wrnexusjs.dev
-# Design Studio uses the gpt-6-luna model at https://api.apimart.ai/v1/chat/completions.
+# Change the model here, then restart the server.
+APIMART_MODEL=gpt-5-mini
+# APIMart endpoint: https://api.apimart.ai/v1/chat/completions.