add JSON-LD structured data

Product pages: Product schema (name, description, image, price/currency,
availability) + BreadcrumbList (Home > Category > Product). Home page:
Organization schema (name, url). Uses Jason with html_safe escaping so
the JSON is safe to embed in <script> tags.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/berrypod_web/components/layouts/shop_root.html.heex

@@ -40,6 +40,11 @@
     <% end %>
     <meta name="twitter:title" content={og_title} />
     <meta name="twitter:description" content={og_description} />
+    <%= if assigns[:json_ld] do %>
+      <script type="application/ld+json">
+        <%= Phoenix.HTML.raw(assigns[:json_ld]) %>
+      </script>
+    <% end %>
     <!-- Preload critical fonts for the current typography preset -->
     <%= for preload <- Berrypod.Theme.Fonts.preload_links(
           @theme_settings.typography,