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

apps/web/app/settings/providers/page.tsx

@@ -1,4 +1,5 @@
 import { AppShell } from "@/components/AppShell";
+import { BackendDownNotice } from "@/components/BackendDownNotice";
 import { ProvidersSettings } from "@/components/settings/ProvidersSettings";
 import { PageHeader } from "@/components/ui/PageHeader";
 import { fetchKimiOauthStatus, fetchProviders } from "@/lib/api/server";
@@ -34,9 +35,7 @@ export default async function ProvidersSettingsPage() {
           description="配置写手、分析、轻量三类能力档位的路由，并管理 API Key 与 Kimi Code OAuth。"
         />
         {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 />
         ) : (
           <ProvidersSettings initial={initial} kimiOauth={kimiOauth} />
         )}