debug(auth): log session cookie source

apps/web/next.config.ts

@@ -1,5 +1,16 @@
 import type { NextConfig } from "next";
 
+const defaultAuthMode = process.env.NODE_ENV === "development" ? "mock" : "real";
+const authMode = (process.env.NEXT_PUBLIC_AUTH_MODE ?? defaultAuthMode).toLowerCase();
+
+if (!["real", "mock"].includes(authMode)) {
+  throw new Error(`Invalid NEXT_PUBLIC_AUTH_MODE "${authMode}". Expected one of: real, mock.`);
+}
+
+if (authMode === "mock" && process.env.NODE_ENV !== "development") {
+  throw new Error("NEXT_PUBLIC_AUTH_MODE=mock is only allowed for local development.");
+}
+
 const nextConfig: NextConfig = {
   transpilePackages: ["@mosaic/ui", "@mosaic/shared"],
 };