Split shared host config into dedicated modules

- Add shared modules for boot, hardware, networking, input, and host defaults
- Move host-specific TOML flags to new option namespaces
- Update SSH and service defaults for the new layout

modules/nixos/shared/networking/default.nix

@@ -1,5 +1,8 @@
 {
-  imports = [./hosts.nix];
+  imports = [
+    ./hosts.nix
+    ./iwd.nix
+  ];
   networking.networkmanager = {
     ethernet.macAddress = "stable";
     wifi.macAddress = "random";

modules/nixos/shared/networking/iwd.nix

@@ -0,0 +1,18 @@
+{
+  config,
+  lib,
+  ...
+}: let
+  inherit (lib) mkEnableOption mkIf;
+  cfg = config.unexplrd.networking.iwd;
+in {
+  options.unexplrd.networking.iwd.enable =
+    mkEnableOption "iwd as the NetworkManager Wi-Fi backend";
+
+  config = mkIf cfg.enable {
+    networking = {
+      networkmanager.wifi.backend = "iwd";
+      wireless.iwd.enable = true;
+    };
+  };
+}