iOS & Android Utility App

I need a single code-base solution that ships to both the App Store and Google Play. The project is a utility-style application, not e-commerce or social media, so the focus is on streamlined functionality rather than heavy UI flourishes.

Core requirements
• Secure user authentication that keeps credentials safe and allows account recovery.
• Real-time push notifications delivered instantly, even when the app is in the background.
• Reliable data synchronisation so a user can pick up on one device exactly where they left off on another.

I’m open to React Native, Flutter, or any other modern cross-platform stack as long as it achieves native-level performance. Clean, well-commented code and a short hand-off session at the end are important to me so I can maintain the product internally.

Please share a brief outline of your proposed architecture and any similar apps you have shipped that demonstrate secure auth, push, and sync. If those align, we can move straight into setting milestones for design, build, internal test, and store submission.