Major config overhaul: use custom modules, setup for multi-host config, and less boilerplate

modules/system/flatpak/default.nix

@@ -0,0 +1,16 @@
+{ lib, config, ... }:
+
+let
+  cfg = config.systemSettings.flatpak;
+in {
+  options = {
+    systemSettings.flatpak = {
+      enable = lib.mkEnableOption "Enable flatpaks";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.flatpak.enable = true;
+    xdg.portal.enable = true;
+  };
+}