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>

assets/js/app.js

@@ -57,6 +57,10 @@ const CartPersist = {
         body: JSON.stringify({items})
       })
     })
+
+    this.handleEvent("persist_country", ({code}) => {
+      document.cookie = `shipping_country=${code};path=/;max-age=${60 * 60 * 24 * 365};SameSite=Lax`
+    })
   }
 }