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.

docs/OPTIONS.md

@@ -75,6 +75,10 @@ Wired in `features/desktop/waybar/default.nix` (filters the battery widget out o
 
 `bool`, default `true` on x86_64. Enables `services.thermald` (Intel thermal daemon). Harmless no-op on AMD; gated off on aarch64.
 
+### `nomarchy.system.desktop.enable`
+
+`bool`, default `nomarchy.system.formFactor == "desktop"`. Desktop preset: pins `powerManagement.cpuFreqGovernor` to `"performance"` (via `mkDefault`) and enables `services.zfs.autoScrub` + `services.zfs.trim` so a future ZFS pool gets sensible maintenance without further config. The ZFS knobs are no-ops until you add `boot.supportedFilesystems = [ "zfs" ]` (plus `networking.hostId`) and a pool. Battery-widget filtering is handled by `formFactor` itself, so this preset doesn't repeat it.
+
 ### `nomarchy.system.containers.enable`
 
 `bool`, default `false`. Rootless Podman with Docker compatibility (`docker` → `podman`), plus `podman-compose`, `podman-tui`, and `dive`.