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/theme_hook.ex

@@ -17,28 +17,6 @@ defmodule BerrypodWeb.ThemeHook do
   alias Berrypod.{Products, Settings, Site, Media}
   alias Berrypod.Theme.{CSSCache, CSSGenerator}
 
-  @default_header_nav [
-    %{"label" => "Home", "href" => "/", "slug" => "home"},
-    %{
-      "label" => "Shop",
-      "href" => "/collections/all",
-      "slug" => "collection",
-      "active_slugs" => ["collection", "pdp"]
-    },
-    %{"label" => "About", "href" => "/about", "slug" => "about"},
-    %{"label" => "Contact", "href" => "/contact", "slug" => "contact"}
-  ]
-
-  @default_footer_nav [
-    %{"label" => "Delivery & returns", "href" => "/delivery", "slug" => "delivery"},
-    %{"label" => "Privacy policy", "href" => "/privacy", "slug" => "privacy"},
-    %{"label" => "Terms of service", "href" => "/terms", "slug" => "terms"},
-    %{"label" => "Contact", "href" => "/contact", "slug" => "contact"}
-  ]
-
-  def default_header_nav, do: @default_header_nav
-  def default_footer_nav, do: @default_footer_nav
-
   def on_mount(:mount_theme, _params, _session, socket) do
     theme_settings = Settings.get_theme_settings()
 
@@ -92,13 +70,11 @@ defmodule BerrypodWeb.ThemeHook do
   end
 
   defp load_header_nav do
-    items = Site.nav_items_for_shop("header")
-    if items == [], do: @default_header_nav, else: add_active_slugs(items)
+    Site.nav_items_for_shop("header") |> add_active_slugs()
   end
 
   defp load_footer_nav do
-    items = Site.nav_items_for_shop("footer")
-    if items == [], do: @default_footer_nav, else: items
+    Site.nav_items_for_shop("footer")
   end
 
   # Add active_slugs for Shop nav item to highlight on collection and pdp pages