fix: Default to a relative path for the admin backend URL (#9947)

packages/admin/dashboard/src/lib/client/client.ts

@@ -1,6 +1,6 @@
 import Medusa from "@medusajs/js-sdk"
 
-export const backendUrl = __BACKEND_URL__ ?? "http://localhost:9000"
+export const backendUrl = __BACKEND_URL__ ?? "/"
 
 export const sdk = new Medusa({
   baseUrl: backendUrl,

packages/admin/dashboard/src/lib/query-client.ts

@@ -1,6 +1,6 @@
 import { QueryClient } from "@tanstack/react-query"
 
-export const MEDUSA_BACKEND_URL = __BACKEND_URL__ ?? "http://localhost:9000"
+export const MEDUSA_BACKEND_URL = __BACKEND_URL__ ?? "/"
 
 export const queryClient = new QueryClient({
   defaultOptions: {