feat: propagate module enables (hyprland/xmonad -> taffybar -> sni)

Move enable propagation into the modules themselves instead of
desktop.nix. Relax the assertion to only prevent both taffybar
and waybar from being enabled simultaneously.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nixos/xmonad.nix

@@ -1,5 +1,7 @@
 { config, pkgs, inputs, makeEnable, ... }:
 makeEnable config "myModules.xmonad" true  {
+  myModules.taffybar.enable = true;
+
   nixpkgs.overlays = with inputs; [
     xmonad.overlay
     xmonad-contrib.overlay