Cross-Platform Mobile App Build

I’m building a brand-new mobile application that must launch on both iOS and Android from day one. The core of the project is a smooth, secure user-authentication flow (email, social, or phone—whichever library you recommend) and a rock-solid in-app purchase pipeline for unlocking premium content. My priority is a single, shared code-base—whether you prefer Flutter, React Native, or another proven cross-platform stack—so future feature updates go live simultaneously on the App Store and Google Play. Clean, modular architecture and well-commented code are must-haves; I plan to maintain the project in-house after launch. Deliverables • Production-ready iOS & Android builds accepted by both stores • Complete source code and build scripts in a public or private repo • Integrated user authentication (secure storage, token refresh, error handling) • Integrated in-app purchases with test products and receipt validation • Brief hand-off documentation explaining project structure and release steps Acceptance Criteria 1. A new user can register, log in, log out, and recover a password without crashes or data leaks. 2. Test purchases succeed on both sandbox environments, and SKU configuration is clearly documented. 3. Both binaries pass store compliance checks with no blocking issues. If additional features such as push notifications become relevant later, the code should be easy to extend, but right now the focus is strictly on authentication and purchases. Let me know your preferred stack, estimated timeline, and any similar apps you’ve shipped that showcase these two features working together seamlessly.