Phase 4: Delete cart/sx/sx_components.py, move renders to sxc/pages

Move all render functions (orders page/rows/oob, order detail/oob,
checkout error, payments panel), header helpers, and serializers from
cart/sx/sx_components.py into cart/sxc/pages/__init__.py. Update all
route imports from sx.sx_components to sxc.pages. Replace
import sx.sx_components in app.py with load_service_components("cart").

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

cart/app.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 import path_setup  # noqa: F401  # adds shared/ to sys.path
-import sx.sx_components as sx_components  # noqa: F401  # ensure Hypercorn --reload watches this file
+from shared.sx.jinja_bridge import load_service_components  # noqa: F401
 
 from decimal import Decimal
 from pathlib import Path
@@ -140,6 +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 shared.sx.handlers import auto_mount_fragment_handlers
     auto_mount_fragment_handlers(app, "cart")