add no-JS fallback for cart country selector

The delivery country form now has action="/cart/country" with a
noscript submit button. Without JS, changing the country and clicking
Update POSTs to a new CartController.update_country action that saves
the country to session and redirects back to /cart.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/berrypod_web/controllers/cart_controller_test.exs

@@ -108,4 +108,19 @@ defmodule BerrypodWeb.CartControllerTest do
       assert cart == []
     end
   end
+
+  describe "POST /cart/country" do
+    test "saves country to session and redirects to /cart", %{conn: conn} do
+      conn = post(conn, ~p"/cart/country", %{"country" => "DE"})
+
+      assert redirected_to(conn) == "/cart"
+      assert get_session(conn, "country_code") == "DE"
+    end
+
+    test "handles missing country param", %{conn: conn} do
+      conn = post(conn, ~p"/cart/country", %{})
+
+      assert redirected_to(conn) == "/cart"
+    end
+  end
 end