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>

system/modules/desktop.nix

@@ -0,0 +1,13 @@
+# Desktop (non-portable) NixOS hosts. Counterpart to ./laptop.nix: imports the
+# shared ./workstation.nix base and swaps the mobile Wi-Fi backend for wired
+# NetworkManager. A desktop host also sets `portable = false` in its host-table
+# entry (flake.nix), which drops the battery block and brightness keybindings
+# from the Sway bar -- see lyrathorpe/home/sway.nix.
+{ ... }:
+{
+  imports = [ ./workstation.nix ];
+
+  # Wired networking. NetworkManager handles DHCP/connections itself; do not
+  # combine with networking.wireless.* (laptop.nix) -- the two backends conflict.
+  networking.networkmanager.enable = true;
+}