Add XDG desktop portal configuration for i3 + KDE integration

Enables proper desktop integration services when using KDE applications
in i3 window manager, including file dialogs, theme integration, and
screen sharing capabilities.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

roles/desktop/programs.nix

@@ -16,5 +16,22 @@ in
     programs.dconf.enable = true;
     services.gnome.gnome-keyring.enable = true;
     programs.kdeconnect.enable = true;
+
+    # XDG Desktop Portal for default application handling in non-KDE environments
+    xdg.portal = {
+      enable = true;
+      extraPortals = with pkgs; [
+        kdePackages.xdg-desktop-portal-kde  # For KDE application integration
+        xdg-desktop-portal-gtk  # Fallback for GTK applications
+      ];
+      config = {
+        common = {
+          default = "kde";
+        };
+        i3 = {
+          default = ["kde" "gtk"];
+        };
+      };
+    };
   };
 }