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/bp/page_admin/routes.py

@@ -47,7 +47,7 @@ def register():
         g.page_config = SimpleNamespace(**raw_pc) if raw_pc else None
 
         from shared.sx.page import get_template_context
-        from sx.sx_components import render_cart_payments_panel
+        from sxc.pages import render_cart_payments_panel
         ctx = await get_template_context()
         html = await render_cart_payments_panel(ctx)
         return sx_response(html)