auto-confirm admin during setup, skip email verification

Setup wizard no longer requires email delivery. Admin account is
auto-confirmed and auto-logged-in via token redirect. Adds setup
secret gate for prod (logged on boot), SMTP env var config in
runtime.exs, email_configured? helper, and admin warning banner
when email isn't set up. Includes plan files for this task and
the follow-up email settings UI.

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

lib/berrypod_web/admin_layout_hook.ex

@@ -26,6 +26,7 @@ defmodule BerrypodWeb.AdminLayoutHook do
       socket
       |> assign(:current_path, "")
       |> assign(:site_live, Settings.site_live?())
+      |> assign(:email_configured, Berrypod.Mailer.email_configured?())
       |> assign(:theme_settings, theme_settings)
       |> assign(:generated_css, generated_css)
       |> Phoenix.LiveView.attach_hook(:set_current_path, :handle_params, fn _params,