fix(android): enable edge-to-edge for Android 15 / API 35 compliance

[Copilot]

Android 15 mandates edge-to-edge for apps targeting SDK 35. The app was missing the explicit opt-in, triggering a Google Play warning.

Changes

• MainActivity.kt — overrides onCreate and calls enableEdgeToEdge() before super.onCreate(), making system bars transparent and providing backward compatibility down to Android 9

class MainActivity : FlutterActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        enableEdgeToEdge()
        super.onCreate(savedInstanceState)
    }
}

No Dart-side changes needed — Flutter's Scaffold, AppBar, and NavigationBar already consume window insets via MediaQuery, so layout is unaffected.

• android/README.md — documents the fix and rationale