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/shop_live/coming_soon.ex

@@ -0,0 +1,22 @@
+defmodule SimpleshopThemeWeb.ShopLive.ComingSoon do
+  use SimpleshopThemeWeb, :live_view
+
+  @impl true
+  def mount(_params, _session, socket) do
+    {:ok, assign(socket, :page_title, "Coming soon")}
+  end
+
+  @impl true
+  def render(assigns) do
+    ~H"""
+    <main class="flex min-h-screen items-center justify-center px-6 text-center" role="main">
+      <div>
+        <h1 class="text-3xl font-bold tracking-tight sm:text-4xl">{@theme_settings.site_name}</h1>
+        <p class="mt-4 text-lg text-[var(--t-text-muted)]">
+          We're getting things ready. Check back soon.
+        </p>
+      </div>
+    </main>
+    """
+  end
+end