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/router.ex

@@ -158,6 +158,7 @@ defmodule BerrypodWeb.Router do
       live "/pages/new", Admin.Pages.CustomForm, :new
       live "/pages/:slug/settings", Admin.Pages.CustomForm, :edit
       live "/pages/:slug", Admin.Pages.Editor, :edit
+      live "/navigation", Admin.Navigation, :index
       live "/media", Admin.Media, :index
       live "/redirects", Admin.Redirects, :index
     end