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/live/admin/product_show.ex

@@ -4,6 +4,7 @@ defmodule BerrypodWeb.Admin.ProductShow do
   alias Berrypod.Products
   alias Berrypod.Products.{Product, ProductImage, ProductVariant}
   alias Berrypod.Cart
+  alias BerrypodWeb.R
 
   @impl true
   def mount(%{"id" => id}, _session, socket) do
@@ -109,7 +110,7 @@ defmodule BerrypodWeb.Admin.ProductShow do
           <.icon name="hero-arrow-top-right-on-square-mini" class="size-4" />
         </.external_link>
         <.link
-          navigate={~p"/products/#{@product.slug}"}
+          navigate={R.product(@product.slug)}
           class="admin-btn admin-btn-ghost admin-btn-sm"
         >
           View on shop <.icon name="hero-arrow-top-right-on-square-mini" class="size-4" />