I want to launch a revenue-focused Android app and the first piece I need built is a rock-solid user-login module. The code can be Kotlin, Java, Flutter, or React Native—whichever delivers smooth performance—but it has to: • handle secure registration and sign-in (email, phone, or a proven third-party auth such as Firebase) • keep users signed in with reliable session persistence • include simple onboarding screens that I can expand later Please target Android 8.0 and above, follow Google Play guidelines, and comment your code so I can extend the project after this milestone. Deliverables are the full, buildable source code and an APK that I can install and test; the job is complete once the login flow runs crash-free on my device.