refactor(nixos): extract shared modules, enable firewall, pin nixpkgs url

- Add common-nixos.nix (timezone, locale, git/fastfetch) imported by every
  NixOS host, and laptop.nix (systemd-boot, sway, dvorak, iwd, firewall)
  imported by X1 and MBP. Strip the nixos-generate-config boilerplate from
  both machine configs and reduce them to host-specific settings.
- Enable the firewall on the laptops (was disabled); X1 opens 22 next to
  its sshd.
- Pin nixpkgs input to github:nixos/nixpkgs/nixos-26.05 for consistency;
  lock rev unchanged (still b51242d).
- Drop unused module arguments.

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

system/modules/laptop.nix

@@ -0,0 +1,21 @@
+# Shared configuration for the physical NixOS laptops (X1, MBP-Asahi). Imported
+# from the host table in flake.nix. Platform-specific bits (bootloader EFI var
+# touching, firmware, audio, hostname, sshd) stay in the per-machine configs.
+{ ... }:
+{
+  boot.loader.systemd-boot.enable = true;
+
+  features.swayDesktop.enable = true;
+
+  console.keyMap = "dvorak";
+
+  # Wi-Fi via iwd with its built-in DHCP/network configuration.
+  networking.wireless.iwd = {
+    enable = true;
+    settings.General.EnableNetworkConfiguration = true;
+  };
+
+  # 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;
+}