feat(nixos): migrate system configuration

configs/nixos/modules/desktop/session/portals.nix

@@ -0,0 +1,23 @@
+{ config, lib, ... }:
+# XDG desktop portal routing for in-app screen sharing and file dialogs.
+let
+  cfg = config.modules.desktop.session.portals;
+in
+{
+  options.modules.desktop.session.portals.enable =
+    lib.mkEnableOption "XDG desktop portals for in-app screen sharing and file dialogs";
+
+  config = lib.mkIf cfg.enable {
+    # The backend packages arrive with the compositor, so only the routing is set
+    # here.
+    # Screen sharing, screenshots, and global shortcuts need the compositor's
+    # own backend.
+    # Everything else uses GTK.
+    xdg.portal.config.common = {
+      default = [ "gtk" ];
+      "org.freedesktop.impl.portal.ScreenCast" = [ "hyprland" ];
+      "org.freedesktop.impl.portal.Screenshot" = [ "hyprland" ];
+      "org.freedesktop.impl.portal.GlobalShortcuts" = [ "hyprland" ];
+    };
+  };
+}