debug(auth): log session cookie source

apps/web/src/app/(authenticated)/layout.tsx

@@ -3,6 +3,7 @@
 import { useEffect } from "react";
 import { useRouter } from "next/navigation";
 import { useAuth } from "@/lib/auth/auth-context";
+import { IS_MOCK_AUTH_MODE } from "@/lib/config";
 import { Navigation } from "@/components/layout/Navigation";
 import { ChatOverlay } from "@/components/chat";
 import type { ReactNode } from "react";
@@ -36,8 +37,18 @@ export default function AuthenticatedLayout({
   return (
     <div className="min-h-screen bg-gray-50">
       <Navigation />
-      <div className="pt-16">{children}</div>
-      <ChatOverlay />
+      <div className="pt-16">
+        {IS_MOCK_AUTH_MODE && (
+          <div
+            className="border-b border-amber-300 bg-amber-50 px-4 py-2 text-sm text-amber-900"
+            data-testid="mock-auth-banner"
+          >
+            Mock Auth Mode (Local Only): Real authentication is bypassed for frontend development.
+          </div>
+        )}
+        {children}
+      </div>
+      {!IS_MOCK_AUTH_MODE && <ChatOverlay />}
     </div>
   );
 }