Add Flatpak packaging

[Dyltom]

Fixes #987.

Summary

• Add a Flatpak manifest for org.ngscopeclient.ngscopeclient using the GNOME 50 runtime.
• Add AppStream metadata installation and a GitHub Actions workflow that builds ngscopeclient.flatpak.
• Make lsb_release optional during CMake configure so the Flatpak SDK can build the project.

Validation

• flatpak-builder-lint manifest packaging/flatpak/org.ngscopeclient.ngscopeclient.yml
• appstreamcli validate --no-net src/ngscopeclient/org.ngscopeclient.ngscopeclient.metainfo.xml
• flatpak-builder --jobs=2 --force-clean --disable-rofiles-fuse --install-deps-from=flathub --repo=.flatpak-build/repo .flatpak-build/build packaging/flatpak/org.ngscopeclient.ngscopeclient.yml
• flatpak build-bundle .flatpak-build/repo .flatpak-build/ngscopeclient.flatpak org.ngscopeclient.ngscopeclient