Launch apps in their own scope instead of the compositor's cgroup (#6541)

* Launch apps in their own scope instead of the compositor's cgroup

The launcher ran desktop entries through gtk-launch, so the app inherited
quickshell's cgroup, which belongs to wayland-wm@hyprland.desktop.service.
A kernel OOM kill there fails the compositor unit and tears down the whole
session, dropping the user at SDDM with every window lost. A single runaway
app took the desktop down three times in one afternoon.

Route launches through uwsm-app so each app gets its own scope under
app-graphical.slice. A runaway app now fails its own scope and the session
keeps running.

The post-install launches had the same inheritance bug in a milder form,
where the app landed in the installer terminal's scope and died with it.
0aedef58 patched that with setsid, which detaches the session but leaves
cgroup membership behind. A scope fixes it properly.

* Detach post-install app launches

* Preserve desktop entry launch compatibility

---------

Co-authored-by: David Heinemeier Hansson <david@hey.com>

bin/omarchy-install-editor-vscode

@@ -26,4 +26,4 @@ printf '{\n  "update.mode": "none"\n}\n' > ~/.config/Code/User/settings.json
 # Apply Omarchy theme to VSCode
 omarchy-theme-set-vscode
 
-setsid gtk-launch code
+setsid uwsm-app -- gtk-launch code >/dev/null 2>&1 &