Launchers: a neutral example app in docs and tests

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.

docs/screenshots.md

@@ -25,7 +25,7 @@ in `…/760/screenshots.vdf`). The JPG is the only copy that stays.
 
 ```nix
 steamFrame.screenshots.enable = true;
-# steamFrame.screenshots.steamUserId = "80511808";   # optional
+# steamFrame.screenshots.steamUserId = "12345678";   # optional
 ```
 
 The account ID is found at runtime: the account last logged in to Steam