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>

offline-backend-20260512/backend/app/gateway/app.py

@@ -37,6 +37,7 @@ from app.gateway.routers import (
     channels,
     checkpoint_migration,
     feedback,
+    dashboard_sessions,
     html_page_favorites,
     intent,
     knowledge,
@@ -1352,6 +1353,9 @@ This gateway provides custom endpoints for models, MCP configuration, skills, an
     # Sensitive-word map (敏感词管理 / 脱敏词) at /api/sensitive-words
     app.include_router(sensitive_words.router)
 
+    # 大屏绘制智能体独立页面会话（按 taskId 存取，不分权）at /api/dashboard-sessions
+    app.include_router(dashboard_sessions.router)
+
     # Standalone parallel multi-agent panel: fan-out streaming + saved tasks
     app.include_router(parallel_agents.router)
     app.include_router(parallel_agent_tasks.router)