* feat(packaging): add self-contained AppImage build
Bundles the wheel plus PyGObject/GTK3/AppIndicator/IBus typelibs into a
relocatable x86_64 AppImage via linuxdeploy + linuxdeploy-plugin-gtk, so
users get a zero-install, no-sandbox-negotiation alternative alongside
AUR/PyPI. Wired into nightly and tagged releases, plus a smoke-test job
in the PR pipeline since AppImage/ELF bundling can't be verified on a
macOS dev machine.
* feat(packaging): add aarch64 AppImage builds
Parameterizes build.sh on uname -m instead of hardcoding x86_64 (both
linuxdeploy and appimagetool publish aarch64 continuous builds, and
pywhispercpp ships manylinux_aarch64 wheels, so no from-source compile
needed). Adds an arch matrix to the PR smoke-test job, and a second
job in nightly.yml/release.yml that builds on GitHub's free
ubuntu-24.04-arm hosted runner and attaches the aarch64 AppImage to
the same release the x86_64 job already created.
