refactor(nixos): extract workstation.nix base from laptop.nix

Move the form-factor-agnostic settings (systemd-boot, swayDesktop, dvorak
console, firewall) into a shared workstation.nix so laptop.nix and the new
desktop.nix can both import them without drifting. laptop.nix keeps only the
iwd Wi-Fi backend.

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

system/modules/workstation.nix

@@ -0,0 +1,15 @@
+# Form-factor-agnostic base for the physical graphical NixOS machines. Imported
+# by both ./laptop.nix and ./desktop.nix; those add only the bits that differ
+# between portable and desktop hosts (chiefly the networking backend).
+{ ... }:
+{
+  boot.loader.systemd-boot.enable = true;
+
+  features.swayDesktop.enable = true;
+
+  console.keyMap = "dvorak";
+
+  # Default-deny inbound. Hosts that run a listening service open their own
+  # ports next to where the service is enabled (e.g. sshd -> 22 on X1).
+  networking.firewall.enable = true;
+}