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/hyprland.nix

@@ -1,5 +1,7 @@
 { config, pkgs, makeEnable, inputs, ... }:
 makeEnable config "myModules.hyprland" true {
+  myModules.taffybar.enable = true;
+
   programs.hyprland = {
     enable = true;
     # Use Hyprland from the flake for proper plugin compatibility