Two issues surfaced while testing the draft inside VirtualBox and on the
ARM64 runner:

1. Fresh Linux systems crashed at startup with
   `error while loading shared libraries: libwebkit2gtk-4.1.so.0: cannot
   open shared object file` because the generated .deb shipped with an
   empty `depends: []` metadata list. dpkg -i would succeed then the
   dynamic linker would fail before main() runs. Now declaring:
     libwebkit2gtk-4.1-0 (>= 2.38), libgtk-3-0,
     libayatana-appindicator3-1, libnotify-bin
   so `sudo apt install ./foo.deb` pulls WebKitGTK automatically. dpkg -i
   users can run `sudo apt -f install` afterwards to resolve.

2. The ARM64 (ubuntu-22.04-arm) bundler step failed at the AppImage
   stage because `/usr/bin/xdg-open` is not preinstalled on that runner.
   Added `xdg-utils` to both release.yml and ci.yml apt installs so both
   architectures build .deb + .rpm + .AppImage cleanly.

README: the Debian/Ubuntu install command is now `apt install ./…`
instead of `dpkg -i` (the former resolves deps, the latter does not),
plus a troubleshooting section for the `libwebkit2gtk-4.1.so.0 cannot
open shared object file` error, and a note about GNOME AppIndicator.