In-App Private Chat Module

I’m adding a lightweight chat feature to my mobile app and need a self-contained modal popup that lets two users message each other privately. The component should:

• Slide in as an overlay within the existing mobile interface (no full-screen takeover).
• Handle text-only messaging—typing, sending, receiving, and auto-scrolling to the latest message.
• Work smoothly on both iOS and Android.
• Expose clean methods so I can trigger the popup from anywhere in the app and pass the target user ID.
• Rely on a simple, real-time backend (Firebase Realtime Database, Firestore, Socket.io, or similar) that you set up and document.

I’ll provide current project files and user authentication details; you supply the chat UI, the real-time sync logic, and concise setup instructions so I can drop it straight into my codebase.