wire Printful shipping rates into cart calculation

Add blueprint_id and print_provider_id to Printful provider_data so the
generic shipping calculator can look up rates. Fix v2 API request format
(order_items key) and response field names. Fetch one representative
variant per product to get accurate per-item rates.

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

lib/simpleshop_theme/clients/printful.ex

@@ -187,7 +187,7 @@ defmodule SimpleshopTheme.Clients.Printful do
       )
   """
   def calculate_shipping(recipient, items) do
-    post("/v2/shipping-rates", %{recipient: recipient, items: items})
+    post("/v2/shipping-rates", %{recipient: recipient, order_items: items})
   end
 
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