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>

test/simpleshop_theme_web/plugs/country_detect_test.exs

@@ -0,0 +1,99 @@
+defmodule SimpleshopThemeWeb.Plugs.CountryDetectTest do
+  use SimpleshopThemeWeb.ConnCase, async: true
+
+  alias SimpleshopThemeWeb.Plugs.CountryDetect
+
+  defp with_cookies(conn) do
+    Plug.Conn.fetch_cookies(conn)
+  end
+
+  describe "call/2" do
+    test "detects GB from en-GB header", %{conn: conn} do
+      conn =
+        conn
+        |> init_test_session(%{})
+        |> with_cookies()
+        |> put_req_header("accept-language", "en-GB,en;q=0.9")
+        |> CountryDetect.call([])
+
+      assert get_session(conn, "country_code") == "GB"
+    end
+
+    test "detects DE from de-DE header", %{conn: conn} do
+      conn =
+        conn
+        |> init_test_session(%{})
+        |> with_cookies()
+        |> put_req_header("accept-language", "de-DE,de;q=0.9,en;q=0.8")
+        |> CountryDetect.call([])
+
+      assert get_session(conn, "country_code") == "DE"
+    end
+
+    test "detects FR from fr-FR header", %{conn: conn} do
+      conn =
+        conn
+        |> init_test_session(%{})
+        |> with_cookies()
+        |> put_req_header("accept-language", "fr-FR,fr;q=0.9")
+        |> CountryDetect.call([])
+
+      assert get_session(conn, "country_code") == "FR"
+    end
+
+    test "defaults to GB when no country in header", %{conn: conn} do
+      conn =
+        conn
+        |> init_test_session(%{})
+        |> with_cookies()
+        |> put_req_header("accept-language", "en;q=0.9")
+        |> CountryDetect.call([])
+
+      assert get_session(conn, "country_code") == "GB"
+    end
+
+    test "defaults to GB when no Accept-Language header", %{conn: conn} do
+      conn =
+        conn
+        |> init_test_session(%{})
+        |> with_cookies()
+        |> CountryDetect.call([])
+
+      assert get_session(conn, "country_code") == "GB"
+    end
+
+    test "does not overwrite existing country_code in session", %{conn: conn} do
+      conn =
+        conn
+        |> init_test_session(%{"country_code" => "US"})
+        |> with_cookies()
+        |> put_req_header("accept-language", "en-GB,en;q=0.9")
+        |> CountryDetect.call([])
+
+      assert get_session(conn, "country_code") == "US"
+    end
+
+    test "cookie overrides session country", %{conn: conn} do
+      conn =
+        conn
+        |> init_test_session(%{"country_code" => "GB"})
+        |> put_req_cookie("shipping_country", "DE")
+        |> with_cookies()
+        |> CountryDetect.call([])
+
+      assert get_session(conn, "country_code") == "DE"
+    end
+
+    test "cookie overrides Accept-Language detection", %{conn: conn} do
+      conn =
+        conn
+        |> init_test_session(%{})
+        |> put_req_cookie("shipping_country", "US")
+        |> with_cookies()
+        |> put_req_header("accept-language", "en-GB,en;q=0.9")
+        |> CountryDetect.call([])
+
+      assert get_session(conn, "country_code") == "US"
+    end
+  end
+end