Build Full Cross-Platform App

I’m looking to have a single code-base application delivered that runs seamlessly on iOS, Android, and the Web. The build must include:

• Secure user authentication so people can register, log in, and recover passwords easily
• Real-time push notifications across all three platforms
• In-app purchase flow that meets Apple, Google Play, and web standards

You’re free to work with React Native, Flutter, or another modern cross-platform stack as long as the final result ships to the App Store, Google Play, and a responsive web host without feature gaps.

At hand-off I need:
1. Source code with clear setup instructions
2. Release-ready builds for each marketplace
3. A brief walkthrough showing the three core features working on every platform

If you’ve launched similar multi-platform apps before, I’d love to hear how you tackled authentication, notifications, and purchases and see a quick demo or link.