fix(web): 后端不可用统一用 BackendDownNotice 替代 404/手搓提示

apps/web/app/page.tsx

@@ -2,6 +2,7 @@ import Link from "next/link";
 import { BookOpen, Plus } from "lucide-react";
 
 import { AppShell } from "@/components/AppShell";
+import { BackendDownNotice } from "@/components/BackendDownNotice";
 import { ProjectLibrary } from "@/components/projects/ProjectLibrary";
 import { EmptyState } from "@/components/ui/EmptyState";
 import { PageHeader } from "@/components/ui/PageHeader";
@@ -38,9 +39,7 @@ export default async function DashboardPage() {
         />
 
         {loadError ? (
-          <p className="rounded border border-conflict bg-panel p-6 text-conflict">
-            无法连接后端服务，请确认 API 已启动后刷新。
-          </p>
+          <BackendDownNotice />
         ) : projects.length === 0 ? (
           <EmptyState
             icon={BookOpen}