Cross-Platform Event Reminder App

I need an intuitive event-reminder app that launches on both iOS and Android. The goal is simple: help users remember every important date while giving them the flexibility to set up notifications exactly the way they like.

Core functionality I must see:
• Real-time event notifications delivered as push alerts.
• Seamless integration with the device’s native calendar so new events appear instantly and existing calendar items can be imported.
• Custom reminder settings—think multiple alerts, snooze intervals, and the option to pick sound or vibration.
• Built-in event sharing so a user can invite friends or colleagues through their preferred messaging or email apps.
I must be able to upload few docs against each customer.

I’m comfortable with modern stacks such as Flutter, React Native or Swift/Kotlin in parallel; feel free to suggest the approach you believe best balances performance with a clean codebase. The final hand-off should include source code, brief setup documentation, and a compiled build for each platform ready for TestFlight and Google Play internal testing.

If you have shipped similar cross-platform utilities before, I’d love to see them and hear how you handled calendar permissions, background notifications, and cross-device syncing.