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/pages/editor.ex

@@ -425,6 +425,8 @@ defmodule BerrypodWeb.Admin.Pages.Editor do
       |> assign(:cart_count, 2)
       |> assign(:cart_subtotal, "£72.00")
       |> assign(:cart_drawer_open, false)
+      |> assign(:header_nav_items, BerrypodWeb.ThemeHook.default_header_nav())
+      |> assign(:footer_nav_items, BerrypodWeb.ThemeHook.default_footer_nav())
       |> preview_page_context(assigns.slug)
 
     extra = Pages.load_block_data(page.blocks, preview)