refactor(hypr): separated exec-once to their corresponding apps

apps/hyprpaper/default.nix

@@ -54,6 +54,12 @@
     exec matugen image "$CACHE_FILE"
   '';
 in {
+  wayland.windowManager.hyprland.settings = {
+    exec-once = [
+      "uwsm app -- hyprpaper"
+    ];
+  };
+
   services.hyprpaper = {
     enable = true;
     settings = {
@@ -81,4 +87,5 @@ in {
   '';
 
   systemd.user.startServices = "sd-switch";
-}
+}
+