fix(desktop): add GTK portal so file dialogs work

Home-manager's Hyprland module enables `xdg.portal` and pins the portal
search dir to the user profile with only the Hyprland backend, which has
no FileChooser implementation. File dialogs in Chromium/Electron apps
(signal, brave) fail with a DBus "No such interface
org.freedesktop.portal.FileChooser" error. Add `xdg-desktop-portal-gtk`
at the home-manager layer (system-level extraPortals aren't in the pinned
dir) and route FileChooser to it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/nixos.nix

@@ -96,7 +96,10 @@
   # so the Secret Service is open without a second passphrase.
   security.pam.services.greetd.enableGnomeKeyring = true;
 
-  # XDG portals (screen sharing, file pickers, and so on).
+  # XDG portals (screen sharing, file pickers, and so on). The GTK portal (for
+  # file dialogs) is added at the home-manager layer, not here: home-manager's
+  # Hyprland module pins the portal search dir to the user profile, so a portal
+  # listed in this system-level extraPortals is never seen. See modules/home.nix.
   xdg.portal = {
     enable = true;
     extraPortals = [ pkgs.xdg-desktop-portal-hyprland ];