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>

lib/berrypod_web/live/admin/pages/editor.ex

@@ -1,7 +1,7 @@
 defmodule BerrypodWeb.Admin.Pages.Editor do
   use BerrypodWeb, :live_view
 
-  alias Berrypod.{LegalPages, Media, Pages, Products}
+  alias Berrypod.{LegalPages, Media, Pages, Products, Site}
   alias Berrypod.Pages.{BlockEditor, BlockTypes, Page}
   alias Berrypod.Products.ProductImage
   alias Berrypod.Theme.{Fonts, PreviewData}
@@ -736,8 +736,8 @@ 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())
+      |> assign(:header_nav_items, load_nav_items("header"))
+      |> assign(:footer_nav_items, load_nav_items("footer"))
       |> preview_page_context(assigns.slug)
 
     extra = Pages.load_block_data(page.blocks, preview)
@@ -853,4 +853,15 @@ defmodule BerrypodWeb.Admin.Pages.Editor do
     |> assign(:save_status, :idle)
     |> assign(:live_region_message, message)
   end
+
+  # Load nav items from database for preview, falling back to defaults if empty
+  defp load_nav_items(location) do
+    case Site.nav_items_for_shop(location) do
+      [] -> default_nav_items(location)
+      items -> items
+    end
+  end
+
+  defp default_nav_items("header"), do: Site.default_header_nav()
+  defp default_nav_items("footer"), do: Site.default_footer_nav()
 end