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/components/layouts.ex

@@ -50,6 +50,10 @@ defmodule BerrypodWeb.Layouts do
     if current_path == "/admin", do: "active", else: nil
   end
 
+  def admin_nav_active?(current_path, "/admin/settings") do
+    if current_path == "/admin/settings", do: "active", else: nil
+  end
+
   def admin_nav_active?(current_path, link_path) do
     if String.starts_with?(current_path, link_path), do: "active", else: nil
   end