The Nix-package launcher example in docs and option examples is now
Signal (pkgs.signal-desktop, desktop ID "signal", sgnl:// and
signalcaptcha:// handlers). The check uses a made-up org.example.App
from a fake package (fake store path), covering the same Exec, action,
MimeType, source and missing-entry cases.
Screenshots: placeholder Steam account IDs in the docs, option example
and check.