fix: heartbeat 500 error, timezone to Beijing, tasks guide collapse

- fix(api): heartbeat endpoint no longer requires request body, fix spa_fallback
  404 handler causing 500 on API routes, add db.refresh() in agent update/heartbeat
- feat(time): unify all frontend time display to Beijing time (Asia/Shanghai)
  via new beijing() utility; backend alert messages also use BJT
- feat(ui): collapse AI agent instruction panel on Tasks page by default
- chore: add backend/app/utils.py and frontend/src/utils/time.js

backend/app/api/agents.py

@@ -50,7 +50,7 @@ async def update_agent(agent_id: int, body: AgentUpdate, db: AsyncSession = Depe
 
 
 @router.post("/{agent_id}/heartbeat", response_model=AgentOut)
-async def heartbeat(agent_id: int, body: AgentHeartbeat, db: AsyncSession = Depends(get_db)):
+async def heartbeat(agent_id: int, db: AsyncSession = Depends(get_db)):
     svc = AgentService(db)
     agent = await svc.heartbeat(agent_id)
     if not agent: