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/(main)/dashboard/page.tsx

@@ -1,8 +1,13 @@
+import type { Metadata } from "next";
 import { auth } from "@/auth";
 import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
 import { Globe, ExternalLink } from "lucide-react";
 import Link from "next/link";
 
+export const metadata: Metadata = {
+  title: "仪表盘",
+};
+
 const SERVER_API_URL = process.env.SERVER_API_URL || "http://backend:8080";
 
 interface Bookmark {
@@ -65,7 +70,7 @@ export default async function DashboardPage() {
             <CardTitle>最近书签</CardTitle>
             {isAdmin && (
               <Link
-                href="/bookmarks"
+                href="/dashboard/bookmarks"
                 className="text-sm text-primary hover:underline"
               >
                 管理 →
@@ -92,6 +97,9 @@ export default async function DashboardPage() {
                         <img
                           src={bm.icon}
                           alt=""
+                          width={16}
+                          height={16}
+                          loading="lazy"
                           className="size-4 object-contain"
                         />
                       ) : (