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

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

@@ -1,7 +1,9 @@
 import { notFound } from "next/navigation";
 
+import { BackendDownNotice } from "@/components/BackendDownNotice";
 import { ForeshadowBoard } from "@/components/foreshadow/ForeshadowBoard";
 import { fetchForeshadow, fetchProject } from "@/lib/api/server";
+import type { ForeshadowView, ProjectResponse } from "@/lib/api/types";
 
 interface PageProps {
   params: Promise<{ id: string }>;
@@ -11,13 +13,30 @@ interface PageProps {
 // ForeshadowBoard（Client）按 status 分四泳道并承载登记/状态变更交互。
 export default async function ForeshadowPage({ 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」。
+  let project: ProjectResponse;
   try {
-    const project = await fetchProject(id);
-    const board = await fetchForeshadow(id);
-    return (
-      <ForeshadowBoard project={project} initialItems={board.foreshadow ?? []} />
-    );
-  } catch {
-    notFound();
+    project = await fetchProject(id);
+  } catch (err) {
+    if (isNotFoundError(err)) notFound();
+    return <BackendDownNotice className="m-6" />;
   }
+
+  // 伏笔看板（GET .../foreshadow）。瞬时错误降级为空看板，不阻塞进页。
+  let initialItems: ForeshadowView[] = [];
+  try {
+    const board = await fetchForeshadow(id);
+    initialItems = board.foreshadow ?? [];
+  } catch {
+    initialItems = [];
+  }
+
+  return <ForeshadowBoard project={project} initialItems={initialItems} />;
+}
+
+// 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");
 }