feat(nixos): add desktop module with non-portable home-manager profile

Add system/modules/desktop.nix (counterpart to laptop.nix): imports the
workstation base and uses wired NetworkManager instead of iwd.

Thread a `portable` flag (default true) through mkHost into specialArgs and
home-manager.extraSpecialArgs, mirroring username/fullName. lyrathorpe/home/
sway.nix consumes it to drop mobile components on desktop hosts:
- status bar swaps the battery block for CPU temperature + network throughput
- screen-brightness keybindings are omitted (no internal backlight)

No host uses desktop.nix yet; a future desktop host imports it and sets
`portable = false`. Verified by evaluating sway.nix both ways:
laptop -> [.. sound battery time] + brightness keys;
desktop -> [.. temperature net sound time], no brightness keys.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

flake.nix

@@ -98,16 +98,34 @@
             fullName,
             modules,
             homeModules,
+            # Host form factor. Laptops inherit the default; a desktop host sets
+            # `portable = false` to drop mobile components (battery block,
+            # brightness keys) from the home-manager Sway config.
+            portable ? true,
           }:
           nixpkgs.lib.nixosSystem {
             inherit system;
-            specialArgs = { inherit inputs username fullName; };
+            specialArgs = {
+              inherit
+                inputs
+                username
+                fullName
+                portable
+                ;
+            };
             modules =
               baseModules
               ++ modules
               ++ [
                 {
-                  home-manager.extraSpecialArgs = { inherit inputs username fullName; };
+                  home-manager.extraSpecialArgs = {
+                    inherit
+                      inputs
+                      username
+                      fullName
+                      portable
+                      ;
+                  };
                   home-manager.users.${username}.imports = homeModules;
                 }
               ];