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>

test/berrypod_web/live/admin/layout_test.exs

@@ -43,10 +43,10 @@ defmodule BerrypodWeb.Admin.LayoutTest do
       assert html =~ user.email
     end
 
-    test "shows view shop and log out links", %{conn: conn} do
+    test "shows shop and log out links", %{conn: conn} do
       {:ok, view, _html} = live(conn, ~p"/admin/orders")
 
-      assert has_element?(view, ~s(a[href="/"]), "View shop")
+      assert has_element?(view, ~s(a[href="/"]), "Shop")
       assert has_element?(view, ~s(a[href="/users/log-out"]), "Log out")
     end
   end