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>

config/config.exs

@@ -102,7 +102,8 @@ config :simpleshop_theme, Oban,
     {Oban.Plugins.Lifeline, rescue_after: :timer.minutes(5)},
     {Oban.Plugins.Cron,
      crontab: [
-       {"*/30 * * * *", SimpleshopTheme.Orders.FulfilmentStatusWorker}
+       {"*/30 * * * *", SimpleshopTheme.Orders.FulfilmentStatusWorker},
+       {"0 */6 * * *", SimpleshopTheme.Sync.ScheduledSyncWorker}
      ]}
   ],
   queues: [images: 2, sync: 1, checkout: 1]