Flutter App Upgrades & Bugfixes

My existing Flutter codebase already builds and runs, but it now needs both extra functionality and a polish pass. The tasks are straightforward:

• Add user authentication that supports secure sign-in and keeps sessions persistent across both Android and iOS.
• Wire up in-app purchases so that users can buy consumables as well as subscriptions inside the app.
• Enable push notifications, ideally through Firebase Cloud Messaging, and show them reliably when the app is in foreground, background, or killed state.
• Resolve performance slow-downs (notably long startup times and sluggish list scrolling) and fix the UI glitches that cause occasional mis-alignment and flicker on some screens.

I will provide the current Git repo and a brief walkthrough of the architecture as soon as we start. Once the work is done I need:

1. Updated source with clean, well-commented commits.
2. Test builds (APK & IPA) demonstrating every new feature.
3. A short changelog describing the optimisations and UI fixes.

Everything should compile cleanly with the latest stable Flutter SDK and run identically on Android and iOS.