improve email settings progressive enhancement and admin layout

- semantic HTML: step numbers inside h2, strong provider names, details
  for adapter configs, strong error messages, fieldset drawer toggle hidden
- inline field errors via flash for no-JS controller fallback
- single form POST button for test email (works with and without JS)
- admin sidebar: remove brand/view-shop, move user email to footer nav
- replace inline style with .admin-setup-step-spaced class
- clean up unused CSS (.admin-brand, .admin-sidebar-header, etc.)

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

lib/berrypod_web/controllers/email_settings_controller.ex

@@ -21,10 +21,9 @@ defmodule BerrypodWeb.EmailSettingsController do
         |> redirect(to: ~p"/admin/settings/email")
 
       {:error, field_errors} when is_map(field_errors) ->
-        message = field_errors |> Map.values() |> Enum.join(". ")
-
         conn
-        |> put_flash(:error, message)
+        |> put_flash(:field_errors, field_errors)
+        |> put_flash(:error_adapter, adapter_key)
         |> redirect(to: ~p"/admin/settings/email")
     end
   end