also refactor nixos modules

modules/nixos/desktop/cosmic.nix

@@ -0,0 +1,25 @@
+{
+  lib,
+  config,
+  inputs,
+  ...
+}: let
+  inherit (lib) mkIf mkEnableOption;
+
+  cfg = config.sneeuwvlok.desktop.cosmic;
+in {
+  options.sneeuwvlok.desktop.cosmic = {
+    enable =
+      mkEnableOption "Enable Cosmic desktop"
+      // {
+        default = config.sneeuwvlok.desktop.use == "cosmic";
+      };
+  };
+
+  config = mkIf cfg.enable {
+    services = {
+      displayManager.cosmic-greeter.enable = true;
+      desktopManager.cosmic.enable = true;
+    };
+  };
+}