partially move macos dock apps to declarative list

modules/common/applications/discord.nix

@@ -11,8 +11,8 @@
 
   config = lib.mkIf (config.gui.enable && config.discord.enable) {
     unfreePackages = [ "discord" ];
+    environment.systemPackages = [ pkgs.discord ];
     home-manager.users.${config.user} = {
-      home.packages = with pkgs; [ discord ];
       xdg.configFile."discord/settings.json".text = ''
         {
           "BACKGROUND_COLOR": "#202225",