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/email_settings_test.exs

@@ -252,7 +252,7 @@ defmodule BerrypodWeb.Admin.EmailSettingsTest do
       assert html =~ "sr-only"
     end
 
-    test "only selected adapter config is visible (hidden attribute)", %{conn: conn} do
+    test "only selected adapter config is open (details element)", %{conn: conn} do
       {:ok, view, _html} = live(conn, ~p"/admin/settings/email")
 
       # Select SMTP
@@ -260,12 +260,12 @@ defmodule BerrypodWeb.Admin.EmailSettingsTest do
       |> form("form[phx-change=\"form_change\"]", %{email: %{adapter: "smtp"}})
       |> render_change()
 
-      # SMTP should not be hidden
-      refute has_element?(view, ~s([data-adapter="smtp"][hidden]))
-      # Others should be hidden
-      assert has_element?(view, ~s([data-adapter="brevo"][hidden]))
-      assert has_element?(view, ~s([data-adapter="sendgrid"][hidden]))
-      assert has_element?(view, ~s([data-adapter="postal"][hidden]))
+      # SMTP details should be open
+      assert has_element?(view, ~s(details[data-adapter="smtp"][open]))
+      # Others should not be open
+      refute has_element?(view, ~s(details[data-adapter="brevo"][open]))
+      refute has_element?(view, ~s(details[data-adapter="sendgrid"][open]))
+      refute has_element?(view, ~s(details[data-adapter="postal"][open]))
     end
   end