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/router.ex

@@ -97,6 +97,7 @@ defmodule BerrypodWeb.Router do
     post "/cart/add", CartController, :add
     post "/cart/remove", CartController, :remove
     post "/cart/update", CartController, :update_item
+    post "/cart/country", CartController, :update_country
   end
 
   # Health check (no auth, no theme loading — for load balancers and uptime monitors)