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}

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" />;
   }
 }

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");
 }

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");
 }

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

@@ -1,5 +1,4 @@
-import { notFound } from "next/navigation";
-
+import { BackendDownNotice } from "@/components/BackendDownNotice";
 import { ToolboxPage } from "@/components/toolbox/ToolboxPage";
 import { fetchProject, fetchToolbox } from "@/lib/api/server";
 
@@ -29,6 +28,6 @@ export default async function ToolboxRoutePage({
       />
     );
   } catch {
-    notFound();
+    return <BackendDownNotice className="m-6" />;
   }
 }

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} />
         )}