feat(nixos): auto-start UxPlay as a user systemd service

Run UxPlay under graphical-session.target so the AirPlay receiver is
discoverable as soon as the GNOME session is up, instead of requiring
a manual `uxplay` invocation per login.

Constraint: UxPlay needs the active graphical session for display + PipeWire
Rejected: system-level service | runs before user session, no display/audio access
Rejected: GNOME autostart .desktop entry | no Restart-on-failure semantics
Confidence: high
Scope-risk: narrow
Directive: keep wantedBy on graphical-session.target (not default.target) so
  the unit only runs when a desktop session is live
Not-tested: behavior on fast user switching between two desktop sessions

configuration.nix

@@ -293,6 +293,20 @@
     };
   };
 
+  # Auto-start UxPlay with the graphical session so iOS Screen Mirroring
+  # can find this host without manually launching it.
+  systemd.user.services.uxplay = {
+    description = "UxPlay AirPlay Mirror Receiver";
+    partOf = [ "graphical-session.target" ];
+    after = [ "graphical-session.target" "pipewire.service" ];
+    wantedBy = [ "graphical-session.target" ];
+    serviceConfig = {
+      ExecStart = "${pkgs.uxplay}/bin/uxplay";
+      Restart = "on-failure";
+      RestartSec = 5;
+    };
+  };
+
   security.pam.services = {
     login.u2fAuth = true;
     sudo.u2fAuth = true;