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

apps/web/app/projects/[id]/codex/page.tsx

@@ -1,5 +1,4 @@
-import { notFound } from "next/navigation";
-
+import { BackendDownNotice } from "@/components/BackendDownNotice";
 import { CodexPage } from "@/components/codex/CodexPage";
 import {
   fetchCharacters,
@@ -37,6 +36,6 @@ export default async function CodexRoutePage({
       />
     );
   } catch {
-    notFound();
+    return <BackendDownNotice className="m-6" />;
   }
 }