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/input/home-row-mods.nix

@@ -0,0 +1,21 @@
+{
+  config,
+  lib,
+  ...
+}: let
+  inherit (lib) mkEnableOption mkIf;
+  cfg = config.unexplrd.input.homeRowMods;
+in {
+  options.unexplrd.input.homeRowMods.enable =
+    mkEnableOption "home row mods for the internal keyboard";
+
+  config = mkIf cfg.enable {
+    services.kanata.enable = true;
+    services.kanata.keyboards.internal = {
+      extraDefCfg = ''
+        process-unmapped-keys no
+      '';
+      configFile = ../kanata/internal.kbd;
+    };
+  };
+}