add data-driven navigation with admin editor

Replace hardcoded header, footer and mobile nav with settings-driven
loops. Nav items stored as JSON via Settings, loaded in ThemeHook with
sensible defaults. New admin navigation editor at /admin/navigation
for add/remove/reorder/save/reset. Mobile bottom nav also driven from
header nav items with icon mapping by slug.

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

lib/berrypod_web/live/admin/theme/index.ex

@@ -399,7 +399,9 @@ defmodule BerrypodWeb.Admin.Theme.Index do
       categories: assigns.preview_data.categories,
       cart_items: PreviewData.cart_drawer_items(),
       cart_count: 2,
-      cart_subtotal: "£72.00"
+      cart_subtotal: "£72.00",
+      header_nav_items: BerrypodWeb.ThemeHook.default_header_nav(),
+      footer_nav_items: BerrypodWeb.ThemeHook.default_footer_nav()
     })
   end