Fix cart load_service_components path

The old sx_components.py used os.path.dirname(__file__) to resolve
the app root. When it was deleted, the replacement call in app.py
used the string "cart" which resolves to /app/cart/ (alembic only),
not /app/ where the sx/ directory lives. Use Path(__file__).parent.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cart/app.py

@@ -140,7 +140,8 @@ def create_app() -> "Quart":
     app.jinja_env.globals["cart_quantity_url"] = lambda product_id: f"/quantity/{product_id}/"
     app.jinja_env.globals["cart_delete_url"] = lambda product_id: f"/delete/{product_id}/"
 
-    load_service_components("cart")
+    from pathlib import Path
+    load_service_components(str(Path(__file__).resolve().parent), service_name="cart")
 
     from shared.sx.handlers import auto_mount_fragment_handlers
     auto_mount_fragment_handlers(app, "cart")