add setup foundations: site gate, registration lockdown, coming soon page

- Settings.site_live?/0 and set_site_live/1 for shop visibility control
- Accounts.has_admin?/0 to detect single-tenant admin existence
- Registration lockdown: /users/register redirects when admin exists
- Setup.setup_status/0 aggregates provider, product, and stripe checks
- Coming soon page at /coming-soon with themed styling
- ThemeHook :require_site_live gate on all public shop routes
  - Site live → everyone through
  - Authenticated → admin preview through
  - No admin → fresh install demo through
  - Otherwise → redirect to coming soon
- Go live / take offline toggle on /admin/settings
- 648 tests, 0 failures

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

test/simpleshop_theme_web/live/user_live/registration_test.exs

@@ -5,13 +5,21 @@ defmodule SimpleshopThemeWeb.UserLive.RegistrationTest do
   import SimpleshopTheme.AccountsFixtures
 
   describe "Registration page" do
-    test "renders registration page", %{conn: conn} do
+    test "renders registration page when no admin exists", %{conn: conn} do
       {:ok, _lv, html} = live(conn, ~p"/users/register")
 
       assert html =~ "Register"
       assert html =~ "Log in"
     end
 
+    test "redirects to login when admin already exists", %{conn: conn} do
+      user_fixture()
+
+      assert {:error,
+              {:redirect, %{to: "/users/log-in", flash: %{"error" => "Registration is closed"}}}} =
+               live(conn, ~p"/users/register")
+    end
+
     test "redirects if already logged in", %{conn: conn} do
       result =
         conn