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/components/core_components.ex

@@ -288,7 +288,7 @@ defmodule BerrypodWeb.CoreComponents do
     ~H"""
     <p class="admin-error" id={@id} role="alert">
       <.icon name="hero-exclamation-circle" class="admin-error-icon" />
-      {render_slot(@inner_block)}
+      <strong>{render_slot(@inner_block)}</strong>
     </p>
     """
   end