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/seo_controller.ex

@@ -2,6 +2,7 @@ defmodule BerrypodWeb.SeoController do
   use BerrypodWeb, :controller
 
   alias Berrypod.{Pages, Products}
+  alias BerrypodWeb.R
 
   def robots(conn, _params) do
     base = BerrypodWeb.Endpoint.url()
@@ -29,23 +30,23 @@ defmodule BerrypodWeb.SeoController do
     categories = Products.list_categories()
 
     static_pages = [
-      {"/", "daily", "1.0"},
-      {"/collections/all", "daily", "0.9"},
-      {"/about", "monthly", "0.5"},
-      {"/contact", "monthly", "0.5"},
-      {"/delivery", "monthly", "0.5"},
-      {"/privacy", "monthly", "0.3"},
-      {"/terms", "monthly", "0.3"}
+      {R.home(), "daily", "1.0"},
+      {R.collection("all"), "daily", "0.9"},
+      {R.about(), "monthly", "0.5"},
+      {R.contact(), "monthly", "0.5"},
+      {R.delivery(), "monthly", "0.5"},
+      {R.privacy(), "monthly", "0.3"},
+      {R.terms(), "monthly", "0.3"}
     ]
 
     category_pages =
       Enum.map(categories, fn cat ->
-        {"/collections/#{cat.slug}", "daily", "0.8"}
+        {R.collection(cat.slug), "daily", "0.8"}
       end)
 
     product_pages =
       Enum.map(products, fn product ->
-        {"/products/#{product.slug}", "weekly", "0.9"}
+        {R.product(product.slug), "weekly", "0.9"}
       end)
 
     custom_pages =