extract setup wizard to dedicated /admin/setup page

Move the setup stepper out of the dashboard into its own LiveView.
Dashboard now redirects to setup when site isn't live, and shows
stats-only view once live. Also cleans up button component variant
handling, fixes alert CSS, and removes stale demo.html.

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

lib/berrypod_web/admin_layout_hook.ex

@@ -8,10 +8,14 @@ defmodule BerrypodWeb.AdminLayoutHook do
     socket =
       socket
       |> assign(:current_path, "")
+      |> assign(:site_live, Berrypod.Settings.site_live?())
       |> Phoenix.LiveView.attach_hook(:set_current_path, :handle_params, fn _params,
                                                                             uri,
                                                                             socket ->
-        {:cont, assign(socket, :current_path, URI.parse(uri).path)}
+        {:cont,
+         socket
+         |> assign(:current_path, URI.parse(uri).path)
+         |> assign(:site_live, Berrypod.Settings.site_live?())}
       end)
 
     {:cont, socket}