aight, making progress

modules/home/application/default.nix

@@ -0,0 +1,26 @@
+{ inputs, config, lib, pkgs, namespace, ... }:
+let
+  inherit (lib) mkIf mkOption;
+  inherit (lib.types) subModule;
+
+  cfg = config.${namespace}.application;
+in
+{
+  options.${namespace}.application = {
+    defaults = mkOption {
+      type = subModule {
+        browser = mkOption {
+          type = enum [ "ladybird" "zen" ];
+          default = "zen";
+          example = "ladybird";
+        };
+      };
+    };
+  };
+
+  config = {
+    home.sessionVariables = {
+      BROWSER = cfg.defaults.browser;
+    };
+  };
+}