integrate R module and add url editor ui

Replaces hardcoded paths with R module throughout:
- Shop components: layout nav, cart, product links
- Controllers: cart, checkout, contact, seo, order lookup
- Shop pages: collection, product, search, checkout success, etc.
- Site context: nav item url resolution

Admin URL management:
- Settings page: prefix editor with validation feedback
- Page renderer: url_editor component for page URLs
- CSS for url editor styling

Test updates for cache isolation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/berrypod_web/controllers/cart_controller.ex

@@ -41,13 +41,13 @@ defmodule BerrypodWeb.CartController do
     conn
     |> Cart.put_in_session(cart)
     |> put_flash(:info, "Added to basket")
-    |> redirect(to: ~p"/cart")
+    |> redirect(to: R.cart())
   end
 
   def add(conn, _params) do
     conn
     |> put_flash(:error, "Could not add item to basket")
-    |> redirect(to: ~p"/cart")
+    |> redirect(to: R.cart())
   end
 
   @doc """
@@ -60,7 +60,7 @@ defmodule BerrypodWeb.CartController do
     conn
     |> Cart.put_in_session(cart)
     |> put_flash(:info, "Removed from basket")
-    |> redirect(to: ~p"/cart")
+    |> redirect(to: R.cart())
   end
 
   @doc """
@@ -73,7 +73,7 @@ defmodule BerrypodWeb.CartController do
 
     conn
     |> Cart.put_in_session(cart)
-    |> redirect(to: ~p"/cart")
+    |> redirect(to: R.cart())
   end
 
   @doc """
@@ -82,11 +82,11 @@ defmodule BerrypodWeb.CartController do
   def update_country(conn, %{"country" => code}) when is_binary(code) and code != "" do
     conn
     |> put_session("country_code", code)
-    |> redirect(to: ~p"/cart")
+    |> redirect(to: R.cart())
   end
 
   def update_country(conn, _params) do
-    redirect(conn, to: ~p"/cart")
+    redirect(conn, to: R.cart())
   end
 
   defp parse_quantity(str) when is_binary(str) do