refactor(modules): declare swayDesktop feature flag in a base module

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.

flake.nix

@@ -114,6 +114,7 @@
         baseModules = [
           ./lyrathorpe/user.nix
           ./system/modules/common-nixos.nix
+          ./system/modules/features.nix
           commonModule
           home-manager.nixosModules.home-manager
           {