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>

test/support/fixtures/orders_fixtures.ex

@@ -0,0 +1,42 @@
+defmodule SimpleshopTheme.OrdersFixtures do
+  @moduledoc """
+  Test helpers for creating orders.
+  """
+
+  alias SimpleshopTheme.Orders
+
+  def order_fixture(attrs \\ %{}) do
+    attrs = Enum.into(attrs, %{})
+
+    items = [
+      %{
+        variant_id: "var_#{System.unique_integer([:positive])}",
+        name: Map.get(attrs, :product_name, "Test product"),
+        variant: Map.get(attrs, :variant_title, "Red / Large"),
+        price: Map.get(attrs, :unit_price, 1999),
+        quantity: Map.get(attrs, :quantity, 1)
+      }
+    ]
+
+    order_attrs = %{
+      items: items,
+      customer_email: Map.get(attrs, :customer_email, "customer@example.com"),
+      currency: Map.get(attrs, :currency, "gbp")
+    }
+
+    {:ok, order} = Orders.create_order(order_attrs)
+
+    case attrs[:payment_status] do
+      "paid" ->
+        {:ok, order} = Orders.mark_paid(order, "pi_test_#{System.unique_integer([:positive])}")
+        order
+
+      "failed" ->
+        {:ok, order} = Orders.mark_failed(order)
+        order
+
+      _ ->
+        order
+    end
+  end
+end