add admin dashboard with setup checklist and stats

Dashboard at /admin shows setup progress (when not live), stat cards
(orders, revenue, products), and recent paid orders table. Replaces
the old AdminController redirect. Add Dashboard to sidebar nav as
first item, update admin bar and theme editor links to /admin.

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

lib/simpleshop_theme/orders.ex

@@ -50,6 +50,16 @@ defmodule SimpleshopTheme.Orders do
     |> Map.new()
   end
 
+  @doc """
+  Returns total revenue (in minor units) from paid orders.
+  """
+  def total_revenue do
+    Order
+    |> where(payment_status: "paid")
+    |> select([o], sum(o.total))
+    |> Repo.one() || 0
+  end
+
   @doc """
   Creates an order with line items from hydrated cart data.