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>

test/support/data_case.ex

@@ -30,15 +30,21 @@ defmodule Berrypod.DataCase do
   setup tags do
     Berrypod.DataCase.setup_sandbox(tags)
     Berrypod.Settings.SettingsCache.invalidate_all()
+    # Clear caches without re-warming from DB (which would bypass sandbox)
+    BerrypodWeb.R.clear()
+    Berrypod.Pages.PageCache.invalidate_all()
+    Berrypod.Redirects.clear_cache()
     :ok
   end
 
   @doc """
   Sets up the sandbox based on the test tags.
+  Returns the owner pid for use in metadata generation.
   """
   def setup_sandbox(tags) do
     pid = Ecto.Adapters.SQL.Sandbox.start_owner!(Berrypod.Repo, shared: not tags[:async])
     on_exit(fn -> Ecto.Adapters.SQL.Sandbox.stop_owner(pid) end)
+    pid
   end
 
   @doc """