add fresh install redirect and admin provider tests

Fresh installs now redirect to /users/register instead of showing the
demo shop. Post-login redirect goes to /admin dashboard (with setup
checklist) instead of /admin/settings. Added 16 provider tests covering
index (list, delete, sync) and form (new, edit, test connection).

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

test/simpleshop_theme_web/controllers/page_controller_test.exs

@@ -1,5 +1,13 @@
 defmodule SimpleshopThemeWeb.PageControllerTest do
-  use SimpleshopThemeWeb.ConnCase
+  use SimpleshopThemeWeb.ConnCase, async: false
+
+  import SimpleshopTheme.AccountsFixtures
+
+  setup do
+    user_fixture()
+    {:ok, _} = SimpleshopTheme.Settings.set_site_live(true)
+    :ok
+  end
 
   test "GET / renders the shop home page", %{conn: conn} do
     conn = get(conn, ~p"/")