fix(quickshell): quit niri on logout instead of terminating the session

loginctl terminate-session kills the login session scope, which holds the
niri-session script, but niri.service runs in the user manager and survives it
-- along with the cleanup line the script would have run next. The compositor
stayed on seat0, greetd returned to the greeter, and every login after that
died immediately. Quitting niri stops niri.service, lets the script finish and
takes graphical-session.target down with it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

defaults/xdg/quickshell/moonarch/PowerPopout.qml

@@ -60,7 +60,7 @@ PopoutPanel {
                 "label": qsTr("Logout"),
                 "icon": 0xf0343, // md-logout
                 "confirm": true,
-                "cmd": ["loginctl", "terminate-session", "auto"]
+                "cmd": ["niri", "msg", "action", "quit", "--skip-confirmation"]
             },
             {
                 "name": "hibernate",