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/controllers/error_html.ex

@@ -94,6 +94,14 @@ defmodule BerrypodWeb.ErrorHTML do
       |> Map.put(:cart_count, 0)
       |> Map.put(:cart_subtotal, "£0.00")
       |> Map.put(:page, page)
+      |> Map.put(
+        :header_nav_items,
+        load_nav("header_nav", &BerrypodWeb.ThemeHook.default_header_nav/0)
+      )
+      |> Map.put(
+        :footer_nav_items,
+        load_nav("footer_nav", &BerrypodWeb.ThemeHook.default_footer_nav/0)
+      )
 
     # Load block data (e.g. products for featured_products block)
     extra = safe_load(fn -> Pages.load_block_data(page.blocks, assigns) end) || %{}
@@ -159,4 +167,11 @@ defmodule BerrypodWeb.ErrorHTML do
       _ -> nil
     end
   end
+
+  defp load_nav(key, default_fn) do
+    case safe_load(fn -> Settings.get_setting(key) end) do
+      items when is_list(items) -> items
+      _ -> default_fn.()
+    end
+  end
 end