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

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

@@ -1,7 +1,9 @@
 import { notFound } from "next/navigation";
 
+import { BackendDownNotice } from "@/components/BackendDownNotice";
 import { OutlineEditor } from "@/components/outline/OutlineEditor";
 import { fetchOutline, fetchProject } from "@/lib/api/server";
+import type { ProjectResponse } from "@/lib/api/types";
 
 interface PageProps {
   params: Promise<{ id: string }>;
@@ -12,13 +14,22 @@ interface PageProps {
 // 仍按需触发 POST .../outline 生成/覆盖所显章节。
 export default async function OutlinePage({ params }: PageProps) {
   const { id } = await params;
+
+  // 仅当项目确实不存在（404）才判 notFound；后端不可用等其它失败降级为「后端不可达」提示，
+  // 不把整页变成「找不到页面」。次级拉取（大纲）的瞬时错误已在 fetchOutline 内降级为空。
+  let project: ProjectResponse;
   try {
-    const project = await fetchProject(id);
-    const initialChapters = await fetchOutline(id);
-    return (
-      <OutlineEditor project={project} initialChapters={initialChapters} />
-    );
-  } catch {
-    notFound();
+    project = await fetchProject(id);
+  } catch (err) {
+    if (isNotFoundError(err)) notFound();
+    return <BackendDownNotice className="m-6" />;
   }
+
+  const initialChapters = await fetchOutline(id);
+  return <OutlineEditor project={project} initialChapters={initialChapters} />;
+}
+
+// fetchProject 抛错信息形如「请求失败 404: ...」；据此区分「项目不存在」与「后端不可用」。
+function isNotFoundError(err: unknown): boolean {
+  return err instanceof Error && err.message.includes("请求失败 404");
 }