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/desktop.nix

@@ -10,14 +10,11 @@ makeEnable config "myModules.desktop" true {
 
   assertions = [
     {
-      assertion = config.myModules.taffybar.enable != config.myModules.waybar.enable;
-      message = "Enable exactly one of myModules.taffybar or myModules.waybar.";
+      assertion = !(config.myModules.taffybar.enable && config.myModules.waybar.enable);
+      message = "myModules.taffybar and myModules.waybar cannot both be enabled.";
     }
   ];
 
-  myModules.taffybar.enable = lib.mkDefault config.myModules.xmonad.enable;
-  myModules.waybar.enable = lib.mkDefault (!config.myModules.xmonad.enable);
-
   services.xserver = {
     exportConfiguration = true;
     enable = true;

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

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

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