feat(dashboard): 大屏绘制智能体独立页面（按 taskId 存取，不分权）

- 新增带导航+侧边栏的独立页面 DashboardAgentPage：按 taskId 取/存会话，
  有有效 report-json 直显大屏(DashboardApp)，否则进生成过程(不自动跳)+输入框可改
- 后端 dashboard_sessions 存储(model/sql/__init__)+路由/by-task(GET/PUT 不分权)
  + deps/app/models 接线；新表靠 create_all 建，无迁移
- 扩展 useStep3Dashboard：buildInitialPrompt(输入任务JSON) + hydrateTranscript(恢复聊天记录)
- 抽出共享 DashboardChatPanel；report-template 增 buildDashboardPromptFromJson
- 输入 seam core/auth/dashboard-input：JSON 接口未就绪先用占位/演示数据兜底
- 路由 StrategyRoutes dashboard-agent + 侧边栏「问答管理→大屏绘制」
- 圆桌业务链卡片「关系图」改横向布局
- 顺带提交：AIWritingContext 登录切换处理、thread-access-error helper

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/core/threads/thread-access-error.ts

@@ -0,0 +1,98 @@
+import { toast } from "sonner";
+
+import { getStoredAuth } from "@/core/auth";
+
+/**
+ * Shared helpers for re-framing LangGraph "thread not accessible" 404s around
+ * the current login identity.
+ *
+ * The most common cause in practice is a login session change: a thread is
+ * owned by the previously logged-in account, so after switching accounts the
+ * backend (``app/gateway/authz.py``) correctly returns
+ * ``HTTP 404: {"detail":"Thread <uuid> not accessible"}``. Raw-toasting that
+ * 404 just leaves the user staring at a scary error. Both the chat stream
+ * (``core/threads/hooks.ts``) and the AI-writing stream
+ * (``open-canvas/contexts/AIWritingContext.tsx``) funnel their stream errors
+ * through here so the UX is identical: detect the login-change 404 and prompt
+ * the user to refresh against the current identity instead of erroring out.
+ */
+
+/** Normalize an unknown stream error into a human-readable message string. */
+export function getStreamErrorMessage(error: unknown): string {
+  if (typeof error === "string" && error.trim()) {
+    return error;
+  }
+  if (error instanceof Error && error.message.trim()) {
+    return error.message;
+  }
+  if (typeof error === "object" && error !== null) {
+    const message = Reflect.get(error, "message");
+    if (typeof message === "string" && message.trim()) {
+      return message;
+    }
+    const nestedError = Reflect.get(error, "error");
+    if (nestedError instanceof Error && nestedError.message.trim()) {
+      return nestedError.message;
+    }
+    if (typeof nestedError === "string" && nestedError.trim()) {
+      return nestedError;
+    }
+  }
+  return "Request failed.";
+}
+
+/**
+ * Detect "thread not found" / forbidden errors from the LangGraph SDK.
+ *
+ * Backend returns 404 both when a thread truly doesn't exist and when the
+ * currently authenticated user doesn't own it ("not found" = no row + no
+ * checkpoint, "not accessible" = row owned by another user / stale login
+ * session). Both mean the current user can't use this thread.
+ */
+export function isThreadNotAccessibleError(message: string): boolean {
+  const lowered = message.toLowerCase();
+  if (/thread\s+[\w-]{8,}\s+not\s+(found|accessible)/i.test(message)) {
+    return true;
+  }
+  // Fallback: a bare 404/403 from the threads endpoint when the SDK has
+  // wrapped the detail differently.
+  return (
+    (lowered.includes("http 404") || lowered.includes("http 403")) &&
+    lowered.includes("thread")
+  );
+}
+
+/** Build a label for the *current* logged-in identity (email → user_id → 未知). */
+export function buildCurrentUserLabel(): string {
+  const auth = getStoredAuth();
+  if (!auth) {
+    return "未登录";
+  }
+  const email = auth.email?.trim();
+  if (email) {
+    return email;
+  }
+  const userId = auth.user_id?.trim();
+  return userId ? userId : "未知用户";
+}
+
+/**
+ * Re-frame a login-change 404 around the current identity and reload the page.
+ *
+ * Shows a friendly toast naming the current login, then full-reloads after a
+ * short delay so the toast is visible and so TanStack Query cache / context
+ * providers / any state captured under the previous user get rebuilt against
+ * the current login. Used by streams whose session id lives in component state
+ * (not the URL) — a plain reload lands them back on a fresh, valid page.
+ */
+export function notifyThreadAccessChangedAndReload(): void {
+  const userLabel = buildCurrentUserLabel();
+  toast.info(
+    `当前登录用户为 ${userLabel}，原会话不存在或无权访问，正在为你刷新页面…`,
+  );
+  if (typeof window !== "undefined") {
+    window.setTimeout(() => {
+      window.location.reload();
+    }, 900);
+  }
+}