itausi/WhatsUp ? reverse-engineered prompt

Build me a WhatsApp style chat app in Flutter with Firebase behind it, for Android and iPhone.

I want it to feel like a modern messaging app with real time chats, a clean home screen, individual conversation pages, and support for sending photos, videos, music, documents, and voice messages. Please include things like emoji support, push notifications, offline access, and a smooth polished interface that feels close to WhatsApp but with its own simple look. Keep messages private and make sure chat data is handled securely, with chats syncing through Firestore and messages not hanging around longer than needed. If anything needs current setup details from the latest Flutter or Firebase docs, go ahead and look them up online. Also make it easy to run, test, and build, with the app looking good on both mobile platforms.