* Add users to seat group when seatd is selected
When a desktop profile uses seatd for seat access, the user must be
in the seat group for the compositor to access input devices. Without
this, sway/hyprland/niri/labwc fail to start after installation.
* Move seat access provisioning into DesktopProfile.provision()
The four Wayland profiles (Hyprland, Sway, niri, labwc) each duplicated a
provision() override calling provision_seat_access(). Move that into the
base DesktopProfile.provision() loop, which already iterates the selected
profiles, and read CustomSetting.SeatAccess from each. The None check now
lives at the call site (walrus), so provision_seat_access() takes a plain
str. This removes the per-profile overrides and their TYPE_CHECKING imports.
* Use top-level imports in seat access utils
The Installer and User imports were under a TYPE_CHECKING guard, but
they cause no circular import (bspwm.py already imports both at top
level). Move them to module scope per review feedback.
091665a975