Hybrid iOS/Android App Design

I need a clean, fully-featured hybrid application that mirrors the look, feel, and core workflow of our existing website while adding the conveniences users expect on mobile. The build must run smoothly on both iOS and Android from a single codebase. Core requirements • Secure user authentication that ties into our current account system (OAuth 2.0 on the backend). • A straightforward in-app purchase flow so customers can buy the same premium content they already purchase on the site. Everything else on the site—navigation, search, content display—should be reproduced with native-feeling UI components and respect our established brand guidelines. Push notifications are not on the roadmap right now; however, future extensibility is welcome, so structure the project so that feature could slot in later without major refactoring. I’m open to React Native, Flutter, or another modern hybrid framework, as long as performance and maintainability are not compromised. Please include: 1. High-fidelity wireframes/prototypes for approval before coding. 2. Well-commented source code and a brief setup guide. 3. Test builds for iOS (TestFlight) and Android (internal track) that pass basic review checks. If you’ve published hybrid apps with secure login and in-app purchases before, I’d love to see examples.