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/simpleshop_theme/orders_test.exs

@@ -0,0 +1,65 @@
+defmodule SimpleshopTheme.OrdersTest do
+  use SimpleshopTheme.DataCase, async: false
+
+  alias SimpleshopTheme.Orders
+
+  import SimpleshopTheme.OrdersFixtures
+
+  describe "list_orders/1" do
+    test "returns all orders" do
+      order1 = order_fixture()
+      order2 = order_fixture()
+
+      orders = Orders.list_orders()
+      order_ids = Enum.map(orders, & &1.id)
+
+      assert order1.id in order_ids
+      assert order2.id in order_ids
+      assert length(orders) == 2
+    end
+
+    test "filters by payment status" do
+      _pending = order_fixture()
+      paid = order_fixture(payment_status: "paid")
+      _failed = order_fixture(payment_status: "failed")
+
+      orders = Orders.list_orders(status: "paid")
+      assert length(orders) == 1
+      assert hd(orders).id == paid.id
+    end
+
+    test "returns all when status is 'all'" do
+      order_fixture()
+      order_fixture(payment_status: "paid")
+
+      orders = Orders.list_orders(status: "all")
+      assert length(orders) == 2
+    end
+
+    test "preloads items" do
+      order_fixture()
+
+      [order] = Orders.list_orders()
+      assert Ecto.assoc_loaded?(order.items)
+      assert length(order.items) == 1
+    end
+  end
+
+  describe "count_orders_by_status/0" do
+    test "returns empty map when no orders" do
+      assert Orders.count_orders_by_status() == %{}
+    end
+
+    test "counts orders by status" do
+      order_fixture()
+      order_fixture()
+      order_fixture(payment_status: "paid")
+      order_fixture(payment_status: "failed")
+
+      counts = Orders.count_orders_by_status()
+      assert counts["pending"] == 2
+      assert counts["paid"] == 1
+      assert counts["failed"] == 1
+    end
+  end
+end