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/components/layouts/admin.html.heex

@@ -118,14 +118,6 @@
                 <.icon name="hero-document" class="size-5" /> Pages
               </.link>
             </li>
-            <li>
-              <.link
-                navigate={~p"/admin/navigation"}
-                class={admin_nav_active?(@current_path, "/admin/navigation")}
-              >
-                <.icon name="hero-bars-3" class="size-5" /> Navigation
-              </.link>
-            </li>
             <li>
               <.link
                 navigate={~p"/admin/media"}