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>

lib/berrypod_web/controllers/cart_controller.ex

@@ -74,6 +74,19 @@ defmodule BerrypodWeb.CartController do
     |> redirect(to: ~p"/cart")
   end
 
+  @doc """
+  Updates delivery country via form POST (no-JS fallback).
+  """
+  def update_country(conn, %{"country" => code}) when is_binary(code) and code != "" do
+    conn
+    |> put_session("country_code", code)
+    |> redirect(to: ~p"/cart")
+  end
+
+  def update_country(conn, _params) do
+    redirect(conn, to: ~p"/cart")
+  end
+
   defp parse_quantity(str) when is_binary(str) do
     case Integer.parse(str) do
       {qty, _} when qty > 0 -> qty