fix(frontend): create NextAuth.js API route handler

NextAuth.js v5 requires an explicit API route to expose authentication endpoints.

Added missing route handler at frontend/src/app/api/auth/[...nextauth]/route.ts that imports and exports handlers from the auth configuration.

This fixes:
- /api/auth/signin endpoint (was 404)
- /api/auth/callback endpoint
- /api/auth/session endpoint
- Frontend authentication flow initialization

Verification: Endpoint now returns HTTP 200 and frontend loads without ClientFetchError.

frontend/src/app/api/auth/[...nextauth]/route.ts

@@ -0,0 +1,3 @@
+import { handlers } from "@/auth/auth"
+
+export const { GET, POST } = handlers