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/open-canvas/contexts/AIWritingContext.tsx

@@ -15,6 +15,11 @@ import {
 import { useStream } from '@langchain/langgraph-sdk/react';
 
 import { getAPIClient } from '@/core/api';
+import {
+  getStreamErrorMessage,
+  isThreadNotAccessibleError,
+  notifyThreadAccessChangedAndReload,
+} from '@/core/threads/thread-access-error';
 
 import { useCanvasGraphContext } from '../contexts/CanvasGraphContext';
 import { saveAIWritingTranscript } from '../api/ai-writing-sessions';
@@ -186,10 +191,18 @@ const AIWritingStreamHost = memo(function AIWritingStreamHost({
     },
     onError(err: unknown) {
       eventBatcherRef.current?.cancel();
-      dispatchRef.current?.({
-        type: 'set_error',
-        message: err instanceof Error ? err.message : String(err),
-      });
+      const message = getStreamErrorMessage(err);
+      // A "thread not accessible/not found" 404 almost always means the writing
+      // thread belongs to a previous login session — the user switched accounts
+      // (or this tab outlived a re-login), so the current account legitimately
+      // can't access the old thread. Instead of dumping the raw HTTP 404 into
+      // the panel, re-frame it around the current identity and reload — same UX
+      // the chat stream gives (see core/threads/hooks.ts).
+      if (isThreadNotAccessibleError(message)) {
+        notifyThreadAccessChangedAndReload();
+        return;
+      }
+      dispatchRef.current?.({ type: 'set_error', message });
     },
     onFinish(finishState) {
       eventBatcherRef.current?.cancel();
@@ -291,7 +304,12 @@ export function AIWritingProvider({ children }: { children: ReactNode }) {
         },
       );
     } catch (err) {
-      dispatch({ type: 'set_error', message: err instanceof Error ? err.message : String(err) });
+      const message = getStreamErrorMessage(err);
+      if (isThreadNotAccessibleError(message)) {
+        notifyThreadAccessChangedAndReload();
+        return;
+      }
+      dispatch({ type: 'set_error', message });
     }
   }, [state.status]);
 
@@ -372,7 +390,12 @@ export function AIWritingProvider({ children }: { children: ReactNode }) {
         streamMode: ['values', 'updates', 'custom', 'messages-tuple'],
       });
     } catch (err) {
-      dispatch({ type: 'set_error', message: err instanceof Error ? err.message : String(err) });
+      const message = getStreamErrorMessage(err);
+      if (isThreadNotAccessibleError(message)) {
+        notifyThreadAccessChangedAndReload();
+        return;
+      }
+      dispatch({ type: 'set_error', message });
     }
   }, [state.sessionId, state.currentPause, state.materials, state.outline,
   state.currentDraftTitle, state.currentDraftMarkdown, state.reviewResult,