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/components/layouts/admin.html.heex

@@ -102,6 +102,14 @@
               <.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"}