Katna for Linux (latest main)

Katna Mail built from main at eabf801 for Linux (x86_64),
version 0.0.0.r1137.geabf801. Replaced on every push
to main. Nothing here is signed. Katna checks for new builds:
it updates the AppImage and the tarball itself, and shows the
command that updates the others. The screenshots show each
package running on its own platform in CI.

• Fedora: sudo dnf install https://github.com/QuakeString/katna/releases/download/linux-latest/katna-x86_64.rpm
• Ubuntu 22.04+ and Debian 12+: download katna_amd64.deb,
  then sudo apt install ./katna_amd64.deb
• Nix: nix profile install github:QuakeString/katna, or
  nix run github:QuakeString/katna
• AppImage: download Katna-x86_64.AppImage, make it
  executable and run it
• Snap: sudo snap install --dangerous katna_amd64.snap, then
  the snap connect lines in packaging/README.md
• Flatpak: flatpak install --user katna-x86_64.flatpak
• Any other Linux: unpack katna-linux-x86_64.tar.gz and run
  its install.sh

See packaging/README.md for more.