add nav editors to Site tab with live preview

- Add header and footer nav editors to Site tab with drag-to-reorder,
  add/remove items, and destination picker (pages, collections, external)
- Live preview updates as you edit nav items
- Remove legacy /admin/navigation page and controller (was saving to
  Settings table, now uses nav_items table)
- Update error_html.ex and pages/editor.ex to load nav from nav_items table
- Update link_scanner to read from nav_items table, edit path now /?edit=site
- Add Site.default_header_nav/0 and default_footer_nav/0 for previews/errors
- Remove fallback logic from theme_hook.ex (database is now source of truth)
- Seed default nav items and social links during setup

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/berrypod_web/live/shop/content_test.exs

@@ -7,6 +7,7 @@ defmodule BerrypodWeb.Shop.ContentTest do
   setup do
     user_fixture()
     {:ok, _} = Berrypod.Settings.set_site_live(true)
+    Berrypod.Site.seed_defaults()
     :ok
   end
 

test/berrypod_web/live/shop/home_test.exs

@@ -8,6 +8,7 @@ defmodule BerrypodWeb.Shop.HomeTest do
   setup do
     user_fixture()
     {:ok, _} = Berrypod.Settings.set_site_live(true)
+    Berrypod.Site.seed_defaults()
 
     conn = provider_connection_fixture()