add admin email settings page with provider selection

Card radio component for picking email providers (SMTP, SendGrid, Mailjet, etc.)
with instant client-side switching via JS hook. Adapter configs are pre-rendered
and toggled without a server round-trip. Secrets are preserved when re-saving
with blank password fields. Includes from address field, test email sending,
and disconnect flow.

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

lib/berrypod_web/router.ex

@@ -176,6 +176,7 @@ defmodule BerrypodWeb.Router do
       live "/providers/new", Admin.Providers.Form, :new
       live "/providers/:id/edit", Admin.Providers.Form, :edit
       live "/settings", Admin.Settings, :index
+      live "/settings/email", Admin.EmailSettings, :index
     end
 
     # Theme editor: admin root layout but full-screen (no sidebar)