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.

system/modules/features.nix

@@ -0,0 +1,12 @@
+# Feature-flag option declarations shared by every NixOS host (imported via
+# baseModules in flake.nix). Declaring the flags here -- rather than inside the
+# module that implements them -- means a host can read or set a flag without
+# importing the (often large) implementation module. In particular,
+# features.swayDesktop.enable is read by lyrathorpe/user.nix on every host, but a
+# headless host (e.g. the Pi) must be able to leave it at its default without
+# pulling in lyrathorpe/swaywm.nix. The implementation lives in swaywm.nix,
+# gated on this flag.
+{ lib, ... }:
+{
+  options.features.swayDesktop.enable = lib.mkEnableOption "the Sway desktop";
+}