Move cart composition from Python to .sx defcomps (Phase 5)

- render_orders_rows: Python loop building row-pairs → ~cart-orders-rows-content
  defcomp that maps over order data and handles pagination sentinel
- render_checkout_error_page: conditional order badge composition →
  ~cart-checkout-error-from-data defcomp
- Remove unused SxExpr import

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

cart/sx/layouts.sx

@@ -116,3 +116,21 @@
 
 (defcomp ~cart-orders-rows (&key rows next-scroll)
   (<> rows next-scroll))
+
+;; Composition defcomp — replaces Python loop in render_orders_rows
+(defcomp ~cart-orders-rows-content (&key orders detail-url-prefix page total-pages next-url)
+  (~cart-orders-rows
+    :rows (map (lambda (od)
+      (~order-row-pair :order od :detail-url-prefix detail-url-prefix))
+      (or orders (list)))
+    :next-scroll (if (< page total-pages)
+      (~infinite-scroll :url next-url :page page
+        :total-pages total-pages :id-prefix "orders" :colspan 5)
+      (~order-end-row))))
+
+;; Composition defcomp — replaces conditional composition in render_checkout_error_page
+(defcomp ~cart-checkout-error-from-data (&key msg order-id back-url)
+  (~checkout-error-content
+    :msg msg
+    :order (when order-id (~checkout-error-order-id :oid (str "#" order-id)))
+    :back-url back-url))