feat: Add Admin UI for banner management

- Create /admin/banners page with full CRUD operations
- Add BannerManager.tsx client component for interactive management
- Add banner-actions.ts server actions (create, update, toggle, delete)
- Add 'Manage Banners' link to admin dashboard
- Features: image preview, activate/deactivate toggle, edit inline

src/app/admin/page.tsx

@@ -117,6 +117,12 @@ export default async function AdminPage() {
                         >
                             Manage Users
                         </Link>
+                        <Link
+                            href="/admin/banners"
+                            className="px-4 py-2 bg-cyan-600 hover:bg-cyan-700 text-white rounded-xl font-bold transition-colors"
+                        >
+                            Manage Banners
+                        </Link>
                         <Link
                             href="/"
                             className="px-4 py-2 bg-zinc-900 dark:bg-zinc-100 text-white dark:text-zinc-900 rounded-xl font-bold hover:bg-zinc-800 transition-colors"