Enable multizone embedding via prefixed asset URLs

frontend/next.config.js

@@ -1,6 +1,28 @@
-/** @type {import('next').NextConfig} */
-const nextConfig = {
-  output: "standalone",
-};
+const { PHASE_DEVELOPMENT_SERVER } = require("next/constants");
 
-module.exports = nextConfig;
+/**
+ * Multizone embedding via assetPrefix.
+ *
+ * The chat is embedded on policyengine.org/uk/chat as a multizone child
+ * (see policyengine-app-v2 PR #1036). Static asset URLs in the generated
+ * HTML get the "/_zones/uk-chat" prefix so the website (policyengine.org)
+ * can proxy them back from the chat host via a Vercel rewrite:
+ *
+ *   /uk/chat                         → chat-host/
+ *   /_zones/uk-chat/_next/:path*     → chat-host/_zones/uk-chat/_next/:path*
+ *
+ * On the chat host, `vercel.json` rewrites the prefixed asset URLs back to
+ * the unprefixed paths Next.js actually serves from. Pages still live at "/"
+ * so the standalone preview URL (policyengine-uk-chat.vercel.app) keeps
+ * working. Matches the household-api-docs zone pattern.
+ *
+ * Dev server runs without the prefix so local development stays simple.
+ */
+module.exports = (phase) => {
+  const isDev = phase === PHASE_DEVELOPMENT_SERVER;
+  /** @type {import('next').NextConfig} */
+  return {
+    output: "standalone",
+    assetPrefix: isDev ? undefined : "/_zones/uk-chat",
+  };
+};

frontend/vercel.json

@@ -0,0 +1,8 @@
+{
+  "rewrites": [
+    {
+      "source": "/_zones/uk-chat/_next/:path*",
+      "destination": "/_next/:path*"
+    }
+  ]
+}