feat(gui): configure Nix daemon scheduling for GUI

Set idle I/O and CPU scheduling policies for the Nix daemon to improve GUI responsiveness. Adjusted scheduling class, priority, and policy to optimize background tasks.

modules/nixos/services/base/default.nix

@@ -43,6 +43,7 @@ in
         [
           age
           bash
+          btop
           cifs-utils
           clevis
           delta

modules/nixos/services/gui/default.nix

@@ -17,6 +17,10 @@ in
   };
 
   config = mkIf cfg.enable {
+    nix.daemonIOSchedClass = "idle"; # best-effort
+    nix.daemonIOSchedPriority = 4; # 7
+    nix.daemonCPUSchedPolicy = "idle"; # batch
+
     services = {
       gnome.localsearch.enable = lib.mkForce false;