feat(system): add desktop preset module

Mirror of the laptop preset for the desktop form factor. New
`nomarchy.system.desktop.enable` defaults to `formFactor == "desktop"`,
so the installer's existing formFactor write auto-flips it on without
installer changes (same pattern as laptop).

The module pins `powerManagement.cpuFreqGovernor` to `"performance"`
(via mkDefault) and enables `services.zfs.{autoScrub,trim}` so a
future ZFS pool gets sensible maintenance for free. The ZFS knobs are
no-ops until the user adds zfs to `boot.supportedFilesystems`.

Battery widget filtering is already driven by `formFactor` itself in
`features/desktop/waybar/default.nix`, so the preset doesn't repeat
it. Closes the "Desktop preset module" Next item.

core/system/options.nix

@@ -108,6 +108,24 @@
       };
     };
 
+    desktop = {
+      enable = lib.mkOption {
+        type = lib.types.bool;
+        default = config.nomarchy.system.formFactor == "desktop";
+        defaultText = lib.literalExpression ''config.nomarchy.system.formFactor == "desktop"'';
+        description = ''
+          Desktop preset: pins `powerManagement.cpuFreqGovernor` to
+          `"performance"` and enables `services.zfs.autoScrub` and
+          `services.zfs.trim` so a future ZFS pool gets sensible
+          maintenance without further config. The ZFS knobs are no-ops
+          until the user adds `boot.supportedFilesystems = [ "zfs" ]`
+          and a pool. Defaults on when `formFactor = "desktop"`. Battery
+          widget filtering is already handled by `formFactor` itself in
+          `features/desktop/waybar/default.nix`.
+        '';
+      };
+    };
+
     containers = {
       enable = lib.mkEnableOption ''
         Rootless Podman with Docker compatibility (`docker` → `podman`),