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/contact_controller.ex

@@ -13,17 +13,17 @@ defmodule BerrypodWeb.ContactController do
       {:ok, _} ->
         conn
         |> put_flash(:info, "Message sent! We'll get back to you soon.")
-        |> redirect(to: ~p"/contact")
+        |> redirect(to: R.contact())
 
       {:error, :invalid_params} ->
         conn
         |> put_flash(:error, "Please fill in all required fields.")
-        |> redirect(to: ~p"/contact")
+        |> redirect(to: R.contact())
 
       {:error, _} ->
         conn
         |> put_flash(:error, "Sorry, something went wrong. Please try again.")
-        |> redirect(to: ~p"/contact")
+        |> redirect(to: R.contact())
     end
   end
 end