packaging(ci): ship Linux AppImage as installable artifact

[ZhuchkaTriplesix]

Parent

Part of #379

Problem

LinuxAppImageInstaller can update .AppImage / zip-containing-AppImage when APPIMAGE is set, but Release CI never builds an AppImage.

Proposal

• Post-process flutter build linux --release bundle into Querya-Desktop-{ver}-linux.AppImage in release.yml.
• Publish alongside the portable linux zip; include in SHA256SUMS.txt.
• Prefer a maintained packaging path (e.g. appimagetool / flutter community recipe) documented in-repo.

Acceptance

• Tagged Release publishes .AppImage
• Running under AppImage updates via in-app updater when asset is present
• Desktop integration via AppImage runtime is smoke-tested on at least one distro
• Docs: Installable Linux = AppImage (plus distro packages from sibling issue)

References

• lib/core/updater/installers/linux_appimage_installer.dart
• lib/core/updater/installers/update_install_context.dart