add shipping costs with live exchange rates and country detection

Shipping rates fetched from Printify during product sync, converted to
GBP at sync time using frankfurter.app ECB exchange rates with 5%
buffer. Cached in shipping_rates table per blueprint/provider/country.

Cart page shows shipping estimate with country selector (detected from
Accept-Language header, persisted in cookie). Stripe Checkout includes
shipping_options for UK domestic and international delivery. Order
shipping_cost extracted from Stripe on payment.

ScheduledSyncWorker runs every 6 hours via Oban cron to keep rates
and exchange rates fresh. REST_OF_THE_WORLD fallback covers unlisted
countries. 780 tests, 0 failures.

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

lib/simpleshop_theme_web/components/page_templates/cart.html.heex

@@ -24,7 +24,13 @@
         </div>
 
         <div>
-          <.order_summary subtotal={@cart_page_subtotal} mode={@mode} />
+          <.order_summary
+            subtotal={@cart_page_subtotal}
+            shipping_estimate={assigns[:shipping_estimate]}
+            country_code={assigns[:country_code] || "GB"}
+            available_countries={assigns[:available_countries] || []}
+            mode={@mode}
+          />
         </div>
       </div>
     <% end %>