nixos/espanso: start with graphical-session.target

On Linux, espanso requires either an X11 or Wayland session to function;
starting with default.target leads to a failed systemd user service that
must be manually restarted.

Starting with graphical-session.target waits until we have something
espanso can work with.

Fixes https://github.com/NixOS/nixpkgs/issues/463578

nixos/modules/services/desktops/espanso.nix

@@ -44,7 +44,7 @@ in
         } daemon";
         Restart = "on-failure";
       };
-      wantedBy = [ "default.target" ];
+      wantedBy = [ "graphical-session.target" ];
     };
 
     environment.systemPackages = [ cfg.package ];