[nobrega8]

Flathub does not accept AI-assisted packaging, so the Flatpak is removed (#183 is closed as not planned). Arch Linux gets a package attached to each release instead, with no repository to host.

• Remove flatpak/ and the Flatpak workflow.
• packaging/arch/PKGBUILD repackages the release's .deb, like the APT repository does.
• The Arch workflow runs after each release: it fills in the version and the SHA-256 of the .deb, builds the package in an archlinux:base-devel container and uploads modbench-x86_64.pkg.tar.zst to the release. On a pull request it only builds. It can also be run by hand for a given tag.
• Install and update with one command, which always takes the latest release:
  sudo pacman -U https://github.com/nobrega8/modbench/releases/latest/download/modbench-x86_64.pkg.tar.zst
• The app detects a copy installed from this package (/var/lib/pacman/local) and shows that command as the update command in Settings, in place of the Flatpak one (17 locales).
• README: install section for Arch.

Only x86_64 is built (the .deb is repackaged, and makepkg labels the package with the architecture of the machine it runs on).