Adds Dockerfile.android (Flutter + Android SDK/JDK 17) and
Dockerfile.linux (Flutter + clang/cmake/GTK3 for desktop builds).
Publish and Scout pipelines now use matrix strategy to build all
three variants in parallel. Registry secrets updated to
REGISTRY_USERNAME/REGISTRY_PASSWORD. Update-check adds explicit
stable channel filter.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>