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/taffybar.nix

@@ -1,5 +1,7 @@
 { config, inputs, pkgs, makeEnable, ... }:
 makeEnable config "myModules.taffybar" false {
+  myModules.sni.enable = true;
+
   nixpkgs.overlays = with inputs; (
     if builtins.isList taffybar.overlays
     then taffybar.overlays