feat: add order management admin with list and detail views

Admin UI at /admin/orders to view, filter, and inspect orders.
Adds list_orders/1 and count_orders_by_status/0 to the Orders
context, status filter tabs, clickable order table with streams,
and a detail page showing items, totals, and shipping address.

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

lib/simpleshop_theme_web/components/layouts/root.html.heex

@@ -45,6 +45,9 @@
           <li>
             <.link href={~p"/admin/theme"}>Theme</.link>
           </li>
+          <li>
+            <.link href={~p"/admin/orders"}>Orders</.link>
+          </li>
           <li>
             <.link href={~p"/admin/settings"}>Credentials</.link>
           </li>