My web app—built entirely with Firebase Studio—has gone down because of a server-side error. I need to locate the fault, patch it, and get the site running smoothly again without data loss. Once the crash is resolved, I want to refine the Razorpay checkout flow: • Add a coupon-code input so a shopper can enter a promo code in real time. • Recalculate and display the discounted amount instantly, then pass that reduced figure to Razorpay so the final charge reflects only the discounted price. The stack currently uses Firebase Hosting, Firestore, and Cloud Functions written in Node.js, while the checkout page relies on Razorpay’s JavaScript SDK.I only need the fixes and enhancements outlined above.