chore(frontend): pin turbopack.root to silence multi-lockfile warning

The repo has lockfiles at the root (Playwright harness) and in
frontend/, so Next.js guesses the workspace root and warns on every
dev-server start. Pin it explicitly.

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

frontend/next.config.ts

@@ -3,6 +3,9 @@ import type { NextConfig } from "next";
 const nextConfig: NextConfig = {
     /* config options here */
     reactCompiler: true,
+    turbopack: {
+        root: __dirname,
+    },
     async rewrites() {
         return [
             {