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>

test/shell.d/app-search-test.sh

@@ -99,9 +99,9 @@ assert(
 )
 
 assert(
-  /function launch\(desktopId, name\) \{[\s\S]*?gtk-launch[\s\S]*?\n  \}/.test(appLibraryQml) &&
-    appLibraryQml.includes('Util.execDetached("gtk-launch "'),
-  'app library runs desktop entry launch through the shell'
+  /function launch\(desktopId, name\) \{[\s\S]*?uwsm-app[\s\S]*?\n  \}/.test(appLibraryQml) &&
+    appLibraryQml.includes('Util.execDetached("uwsm-app -- gtk-launch "'),
+  'app library launches desktop entries through gtk-launch in their own scope'
 )
 
 assert(