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>

lib/simpleshop_theme_web/live/user_live/registration.ex

@@ -48,9 +48,15 @@ defmodule SimpleshopThemeWeb.UserLive.Registration do
   end
 
   def mount(_params, _session, socket) do
-    changeset = Accounts.change_user_email(%User{}, %{}, validate_unique: false)
-
-    {:ok, assign_form(socket, changeset), temporary_assigns: [form: nil]}
+    if Accounts.has_admin?() do
+      {:ok,
+       socket
+       |> put_flash(:error, "Registration is closed")
+       |> redirect(to: ~p"/users/log-in")}
+    else
+      changeset = Accounts.change_user_email(%User{}, %{}, validate_unique: false)
+      {:ok, assign_form(socket, changeset), temporary_assigns: [form: nil]}
+    end
   end
 
   @impl true