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/plugs/country_detect.ex

@@ -0,0 +1,55 @@
+defmodule SimpleshopThemeWeb.Plugs.CountryDetect do
+  @moduledoc """
+  Plug that detects the visitor's country from cookies or Accept-Language.
+
+  Priority:
+  1. `shipping_country` cookie (set when user explicitly changes country)
+  2. Accept-Language header (locale tags like `en-GB` → `GB`)
+  3. Falls back to `"GB"`
+
+  The result is stored in the session as `country_code` so LiveViews can
+  read it. Only runs once per session — skips if `country_code` is already
+  set (unless the cookie has changed).
+  """
+
+  import Plug.Conn
+
+  @default_country "GB"
+  @cookie_name "shipping_country"
+
+  def init(opts), do: opts
+
+  def call(conn, _opts) do
+    cookie_country = conn.cookies[@cookie_name]
+    session_country = get_session(conn, "country_code")
+
+    cond do
+      # Cookie takes priority — user explicitly chose this country
+      cookie_country not in [nil, ""] and cookie_country != session_country ->
+        put_session(conn, "country_code", cookie_country)
+
+      # Session already set and no cookie override
+      session_country != nil ->
+        conn
+
+      # First visit: detect from Accept-Language
+      true ->
+        country = detect_from_header(conn)
+        put_session(conn, "country_code", country)
+    end
+  end
+
+  defp detect_from_header(conn) do
+    conn
+    |> get_req_header("accept-language")
+    |> List.first("")
+    |> parse_country()
+  end
+
+  defp parse_country(header) do
+    case Regex.run(~r/[a-z]{2}-([A-Z]{2})/, header) do
+      [_, country] -> country
+      nil -> @default_country
+    end
+  end
+end