* 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.
