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/checkout_controller.ex

@@ -11,7 +11,7 @@ defmodule BerrypodWeb.CheckoutController do
     unless Settings.has_secret?("stripe_api_key") do
       conn
       |> put_flash(:error, "Checkout isn't available yet")
-      |> redirect(to: ~p"/cart")
+      |> redirect(to: R.cart())
     else
       cart_items = Cart.get_from_session(get_session(conn))
       hydrated = Cart.hydrate(cart_items)
@@ -20,12 +20,12 @@ defmodule BerrypodWeb.CheckoutController do
         hydrated == [] ->
           conn
           |> put_flash(:error, "Your basket is empty")
-          |> redirect(to: ~p"/cart")
+          |> redirect(to: R.cart())
 
         Enum.any?(hydrated, &(&1.is_available == false)) ->
           conn
           |> put_flash(:error, "Some items in your basket are no longer available")
-          |> redirect(to: ~p"/cart")
+          |> redirect(to: R.cart())
 
         true ->
           track_checkout_start(conn)
@@ -45,7 +45,7 @@ defmodule BerrypodWeb.CheckoutController do
 
         conn
         |> put_flash(:error, "Something went wrong. Please try again.")
-        |> redirect(to: ~p"/cart")
+        |> redirect(to: R.cart())
     end
   end
 
@@ -67,14 +67,12 @@ defmodule BerrypodWeb.CheckoutController do
         }
       end)
 
-    base_url = BerrypodWeb.Endpoint.url()
-
     params =
       %{
         mode: "payment",
         line_items: line_items,
-        success_url: "#{base_url}/checkout/success?session_id={CHECKOUT_SESSION_ID}",
-        cancel_url: "#{base_url}/cart",
+        success_url: R.url(R.checkout_success()) <> "?session_id={CHECKOUT_SESSION_ID}",
+        cancel_url: R.url(R.cart()),
         metadata: %{"order_id" => order.id},
         shipping_address_collection: %{
           allowed_countries: ["GB", "US", "CA", "AU", "DE", "FR", "NL", "IE", "AT", "BE"]
@@ -96,7 +94,7 @@ defmodule BerrypodWeb.CheckoutController do
 
         conn
         |> put_flash(:error, "Payment setup failed. Please try again.")
-        |> redirect(to: ~p"/cart")
+        |> redirect(to: R.cart())
 
       {:error, reason} ->
         Logger.error("Stripe session creation failed: #{inspect(reason)}")
@@ -104,7 +102,7 @@ defmodule BerrypodWeb.CheckoutController do
 
         conn
         |> put_flash(:error, "Payment setup failed. Please try again.")
-        |> redirect(to: ~p"/cart")
+        |> redirect(to: R.cart())
     end
   end