[Stability][BUG-009] Chat history timestamps are timezone-shifted

Parent stability dashboard: #3107

This issue tracks BUG-009 from #3107.

Problem

Recently completed chats can appear in the history/search list as roughly 8 hours ago when tested in Asia/Shanghai.

Representative API shape:

Source: threads/history API response inspected in the browser/runtime.

{
  "created_at": "2026-05-20T06:10:22.970977",
  "updated_at": "2026-05-20T06:12:31.333753"
}

These timestamps have no timezone suffix such as Z or +00:00.

Mechanism

Browser JavaScript parses timezone-less ISO strings as local time:

new Date("2026-05-20T06:12:31.333753")

In Asia/Shanghai, this is interpreted as local 06:12, not UTC 06:12. If the backend intended UTC, the displayed relative time is shifted by about 8 hours.

Code evidence

frontend/src/app/workspace/chats/page.tsx
frontend/src/core/utils/datetime.ts

The frontend passes the raw timestamp string into date formatting without normalizing timezone-less backend timestamps.

Impact

• Recent threads look stale.
• History/search ordering and user trust in persistence/status are affected.

Expected behavior

Backend should return timezone-aware ISO timestamps, preferably UTC with Z, for example:

2026-05-20T06:12:31.333753Z

Alternatively, the frontend should normalize DeerFlow API timestamps without timezone as UTC before formatting.