Updates the Gradle namespace/applicationId, moves MainActivity.kt to the
matching source directory, and refreshes the package id in the README.
The Dart package name in pubspec.yaml is unchanged. Since the application
id changed, existing installs will not upgrade in place and their local
secure-storage/prefs data does not carry over.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
