lyrathorpe/user.nix reads features.swayDesktop.enable on every host, but the
option was declared inside lyrathorpe/swaywm.nix -- so a host that does not
import swaywm.nix (a headless server) would fail evaluation. Move the option
declaration to a new always-imported system/modules/features.nix and wire it
into baseModules; swaywm.nix keeps only its implementation (config) block.
Headless hosts can now omit swaywm.nix and the flag defaults to false.
Emma Thorpe