frontend: rebuild bookmark page with drag-and-drop, search, and theme system

- bookmark management with dnd-kit reordering, bulk edit, search,
  category filter/rename, and meta auto-fetch
- migrate /bookmarks → /dashboard/bookmarks under (main) layout
- homepage redesign with category grid, /-key search, dock tooltips
- theme toggle + use-theme, sonner toasts, alert-dialog/skeleton,
  visual refresh of auth pages

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

frontend/app/unauthorized/page.tsx

@@ -1,11 +1,21 @@
+import type { Metadata } from "next";
+import Link from "next/link";
+
+export const metadata: Metadata = {
+  title: "无权访问",
+};
+
 export default function UnauthorizedPage() {
   return (
-    <div className="flex min-h-screen flex-col items-center justify-center p-4">
-      <h1 className="text-3xl font-bold">403</h1>
-      <p className="mt-2 text-gray-500">您没有权限访问该页面</p>
-      <a href="/dashboard" className="mt-4 text-blue-600 hover:underline">
+    <div className="flex min-h-screen flex-col items-center justify-center bg-gradient-to-br from-background via-background to-muted/40 p-4 text-center">
+      <h1 className="text-5xl font-bold tracking-tight">403</h1>
+      <p className="mt-3 text-muted-foreground">您没有权限访问该页面</p>
+      <Link
+        href="/dashboard"
+        className="mt-6 text-sm font-medium text-primary underline-offset-4 hover:underline"
+      >
         返回仪表盘
-      </a>
+      </Link>
     </div>
   );
 }