build.sh was producing unsigned APKs because it reimplemented the Docker
build inline without the signing step from build-tauri-android.sh. Now
uses the same pipeline: find keystore (release preferred, debug fallback),
zipalign -f 4, apksigner sign with keystore credentials.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>