Labwc service (#2248)

* services: add LabwcService with quit

labwc has a minimal IPC surface (no socket, no queries) but it does
expose `labwc --exit` as a clean shutdown path. Wrap that in a small
Singleton service following the same shape as DwlService/NiriService
so the compositor-specific dispatch in callers can stay uniform.

* session: dispatch labwc logout via LabwcService

CompositorService.isLabwc was detected but never dispatched in
_logout(); labwc sessions therefore fell through to the Hyprland
exit call, which silently no-ops under labwc. Users had to set
customPowerActionLogout to 'labwc --exit' as a workaround.

Add a labwc branch alongside the existing niri/dwl/sway branches
so the power menu logout works out of the box.

quickshell/Services/LabwcService.qml

@@ -0,0 +1,15 @@
+pragma Singleton
+pragma ComponentBehavior: Bound
+
+import QtQuick
+import Quickshell
+
+Singleton {
+    id: root
+
+    // Exit the labwc session. Used by SessionService when the user
+    // triggers logout and no custom logout command is configured.
+    function quit() {
+        Quickshell.execDetached(["labwc", "--exit"]);
+    }
+}

quickshell/Services/SessionService.qml

@@ -314,6 +314,11 @@ Singleton {
                 return;
             }
 
+            if (CompositorService.isLabwc) {
+                LabwcService.quit();
+                return;
+            }
+
             if (CompositorService.isSway || CompositorService.isScroll || CompositorService.isMiracle) {
                 try {
                     I3.dispatch("exit");