chore(proxy): expose backend under /admin139 on frontend port

frontend/next.config.ts

@@ -4,15 +4,15 @@ const nextConfig: NextConfig = {
   // For local dev convenience. In production you should configure reverse proxy
   // and set NEXT_PUBLIC_API_BASE to your public API origin.
   async rewrites() {
-    // If the user sets NEXT_PUBLIC_API_BASE to "/api", we can proxy to backend
-    // from the Next.js server (SSR). This does NOT affect browser fetch.
+    // Reverse proxy backend under a path prefix, so browser can access backend
+    // with same-origin (no CORS): http://<host>:7888/admin139/...
     const backendInternal = process.env.BACKEND_INTERNAL_URL;
     if (!backendInternal) return [];
 
     return [
       {
-        source: "/api/:path*",
-        destination: `${backendInternal}/api/:path*`,
+        source: "/admin139/:path*",
+        destination: `${backendInternal}/:path*`,
       },
     ];
   },