* fix: enable unrar on all distros, not just Fedora
unrar was gated behind distro.id() == 'fedora', so on Debian/Ubuntu
the binary was installed but never invoked. Try unrar unconditionally
after 7z, with an optional rarfile Python fallback.
Fixes CBR extraction on non-Fedora Linux distributions.
* fix: gate unrar on Linux and drop unused distro dependency
Debian/Ubuntu's '7zip' package ships the upstream 7zz binary, which
excludes RAR support for licensing reasons, so 7z fails to read .cbr
(RAR) archives. unrar was installed but only invoked when
distro.id() == 'fedora', so on Debian it was never tried and extraction
failed.
Per maintainer feedback, gate unrar on platform.system() == 'Linux'
(not needed on Windows or macOS) and drop the rarfile PyPI fallback to
avoid a new dependency. Since the code no longer imports distro, also
remove it from the requirements files and setup.py.
Nils Leo