frontend: add bookmark management and homepage navigation

Admin-only /bookmarks page for managing entries; homepage now renders
public bookmarks as a category-grouped navigation grid (empty state
links admin to the manager). Dashboard gains a recent-bookmarks card,
dock and main layout get a bookmark entry for admins, and the
middleware protects /bookmarks.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/app/(main)/layout.tsx

@@ -22,6 +22,11 @@ export default async function MainLayout({
             <Link href="/dashboard" className="text-sm hover:underline">
               仪表盘
             </Link>
+            {user?.role === "admin" && (
+              <Link href="/bookmarks" className="text-sm hover:underline">
+                书签
+              </Link>
+            )}
             <form
               action={async () => {
                 "use server";