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-and-launch

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-# omarchy:summary=Install a packaged app and gtk-launch it once it finishes
+# omarchy:summary=Install a packaged app and launch it once it finishes
 # omarchy:args=<display-name> <packages> <desktop-id>
 # omarchy:examples=omarchy install and launch Cursor cursor-bin cursor
 
@@ -13,5 +13,9 @@ if [[ -z $name || -z $packages || -z $desktop_id ]]; then
   exit 1
 fi
 
+printf -v install_message '%q' "Installing ${name}..."
+printf -v desktop_id_arg '%q' "$desktop_id"
+
+# The subshell keeps & from backgrounding the package installation too.
 exec omarchy-launch-floating-terminal-with-presentation \
-  "echo 'Installing ${name}...'; omarchy-pkg-add ${packages} && setsid gtk-launch ${desktop_id}"
+  "echo ${install_message}; omarchy-pkg-add ${packages} && (setsid uwsm-app -- gtk-launch ${desktop_id_arg} >/dev/null 2>&1 &)"