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/controllers/email_settings_controller_test.exs

@@ -33,14 +33,18 @@ defmodule BerrypodWeb.EmailSettingsControllerTest do
       assert Settings.get_setting("email_adapter") == "brevo"
     end
 
-    test "redirects with error on validation failure", %{conn: conn} do
+    test "redirects with field errors on validation failure", %{conn: conn} do
       conn =
         post(conn, ~p"/admin/settings/email", %{
           email: %{adapter: "brevo", brevo: %{api_key: ""}}
         })
 
       assert redirected_to(conn) =~ ~p"/admin/settings/email"
-      assert Phoenix.Flash.get(conn.assigns.flash, :error) =~ "required"
+
+      field_errors = Phoenix.Flash.get(conn.assigns.flash, :field_errors)
+      assert is_map(field_errors)
+      assert field_errors |> Map.values() |> Enum.any?(&String.contains?(&1, "required"))
+      assert Phoenix.Flash.get(conn.assigns.flash, :error_adapter) == "brevo"
     end
   end