Add security headers and proxy error handling to Hono server

Every response gets X-Frame-Options, X-Content-Type-Options, and
Referrer-Policy (SEC-07/FE-08). Backend proxy failures now return a
logged 502 instead of an opaque uncaught error (FE-23).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/server.ts

@@ -324,6 +324,27 @@ class SuppressRenderToolTextMiddleware extends (Middleware as any) {
 
 const app = new Hono();
 
+// Security headers on every response. Responses returned by fetch() (the
+// backend proxy) carry immutable headers, so rebuild the Response instead of
+// mutating in place.
+const SECURITY_HEADERS: Record<string, string> = {
+  "X-Frame-Options": "DENY",
+  "X-Content-Type-Options": "nosniff",
+  "Referrer-Policy": "strict-origin-when-cross-origin",
+};
+
+app.use("*", async (c, next) => {
+  await next();
+  const res = c.res;
+  const headers = new Headers(res.headers);
+  for (const [k, v] of Object.entries(SECURITY_HEADERS)) headers.set(k, v);
+  c.res = new Response(res.body, {
+    status: res.status,
+    statusText: res.statusText,
+    headers,
+  });
+});
+
 app.all("/api/copilotkit/*", async (c) => {
   const cookieHeader = c.req.header("cookie") ?? "";
   const match = cookieHeader.match(/(?:^|;\s*)ws_token=([^;]+)/);
@@ -398,7 +419,12 @@ const proxyToBackend = async (c: import("hono").Context) => {
     // @ts-expect-error undici requires duplex for streamed bodies
     init.duplex = "half";
   }
-  return fetch(target, init);
+  try {
+    return await fetch(target, init);
+  } catch (err) {
+    console.error(`[proxy] ${method} ${url.pathname} → backend failed:`, err);
+    return c.json({ error: "Backend unavailable" }, 502);
+  }
 };
 
 app.all("/api/v1/*", proxyToBackend);