Launchers: neutral examples and made-up fixtures

Examples showed specific apps as if the repository installed or
configured them.

- Docs, README, template and option examples: the Signal Flatpak
  (org.signal.Signal, sgnl:// and signalcaptcha://) and a made-up
  org.example.App instead of the previous chat and remote-desktop apps;
  no install lines (the app is installed by the user, as noted).
- Portal fix: the file-dialog symptom phrased generically.
- Launcher check: made-up Flatpak entries org.example.Chat (Electron,
  --file-forwarding @@u %U @@, an action, an SSO scheme) and
  org.example.Remote (Qt, -qwindowtitle %c @@u %u @@, localized keys)
  replace the copied ones; Firefox, gedit and Jellyfin stay.

template/home.nix

@@ -65,11 +65,11 @@
   #   # org.jellyfin.JellyfinDesktop yourself); gives it devices=all:
   #   jellyfin.hardwareDecoding.enable = true;
   #   # Apps that keep logins in the KDE wallet: one wallet for both
-  #   # sessions (Element: install im.riot.Riot yourself):
-  #   launchers."im.riot.Riot" = {
+  #   # sessions (install the Signal Flatpak org.signal.Signal yourself):
+  #   launchers."org.signal.Signal" = {
   #     keyring = { enable = true; electron = true; };
-  #     # SSO callback:
-  #     defaultFor = [ "x-scheme-handler/element" "x-scheme-handler/io.element.desktop" ];
+  #     # link callbacks:
+  #     defaultFor = [ "x-scheme-handler/sgnl" "x-scheme-handler/signalcaptcha" ];
   #   };
   #   docker.enable = true;              # rootless Docker, user service
   # };