add admin UX polish: nav grouping, inline settings, real preview data

- sidebar nav grouped under Shop/Content/Settings section headers with
  subtle uppercase labels (#105)
- custom page settings now show inline in a collapsible panel within the
  editor instead of navigating away to a separate page (#107)
- admin editor preview loads real products and categories from the DB,
  falling back to PreviewData only on fresh installs (#108)

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

test/berrypod_web/live/admin/pages_test.exs

@@ -987,7 +987,7 @@ defmodule BerrypodWeb.Admin.PagesTest do
     test "shows settings button for custom pages", %{conn: conn} do
       {:ok, view, _html} = live(conn, ~p"/admin/pages/size-guide")
 
-      assert has_element?(view, "a[href='/admin/pages/size-guide/settings']", "Settings")
+      assert has_element?(view, "button", "Settings")
     end
 
     test "does not show reset to defaults for custom pages", %{conn: conn} do