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/orders.ex

@@ -10,6 +10,41 @@ defmodule SimpleshopTheme.Orders do
   alias SimpleshopTheme.Repo
   alias SimpleshopTheme.Orders.{Order, OrderItem}
 
+  @doc """
+  Lists orders, optionally filtered by payment status.
+
+  ## Options
+
+    * `:status` - filter by payment_status ("paid", "pending", "failed", "refunded")
+      Pass nil or "all" to return all orders.
+
+  Returns orders sorted by newest first, with items preloaded.
+  """
+  def list_orders(opts \\ []) do
+    status = opts[:status]
+
+    Order
+    |> maybe_filter_status(status)
+    |> order_by([o], desc: o.inserted_at)
+    |> preload(:items)
+    |> Repo.all()
+  end
+
+  defp maybe_filter_status(query, nil), do: query
+  defp maybe_filter_status(query, "all"), do: query
+  defp maybe_filter_status(query, status), do: where(query, [o], o.payment_status == ^status)
+
+  @doc """
+  Returns a map of payment_status => count for all orders.
+  """
+  def count_orders_by_status do
+    Order
+    |> group_by(:payment_status)
+    |> select([o], {o.payment_status, count(o.id)})
+    |> Repo.all()
+    |> Map.new()
+  end
+
   @doc """
   Creates an order with line items from hydrated cart data.