The r0adkll/sign-android-release@v1 hardcodes build-tools 29.0.3 which
isn't available. Now uses apksigner from the latest installed build-tools
directly with zipalign + sign + verify steps.
The gradlew stub and missing gradle-wrapper.jar caused CI build to fail.
Now uses gradle/actions/setup-gradle@v4 to install Gradle, then generates
wrapper before building. Also renames signed APK and uploads both signed
and unsigned to GitHub Releases.