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/live/shop/cart.ex

@@ -5,7 +5,7 @@ defmodule SimpleshopThemeWeb.Shop.Cart do
 
   @impl true
   def mount(_params, _session, socket) do
-    {:ok, assign(socket, :page_title, "Cart")}
+    {:ok, assign(socket, page_title: "Cart")}
   end
 
   @impl true