fix: reorder chat history sidebar after new message in existing chat

[hztBUAA]

Summary

Fixes #2763

When sending a new message in an existing (resumed) chat, the chat history sidebar doesn't reorder to move the active chat to the top. This happens because the first_interaction socket event only fires once per session — so when a user resumes a thread and sends a message, no thread list refresh is triggered.

Changes

• Added a useEffect in ThreadHistory that tracks message count changes
• When a new user_message is detected in the current thread, the thread's createdAt is updated to the current time
• This triggers groupByDate to re-sort the thread list, moving the active conversation to the top

How it works

The fix watches messages.length and compares it to the previous count via a ref. When:

1. The message count increases (new message added)
2. There were already messages (not the initial load)
3. The latest message is a user_message
4. The current thread exists in the history list and is not already at the top

...the thread's createdAt timestamp is updated, which causes the existing groupByDate sorting (in the threadHistoryState atom effect) to reposition the thread to the top of the "Today" group.

Test plan

• Open an existing chat from the sidebar history
• Send a new message in that chat
• Verify the chat moves to the top of the history list immediately
• Create a new chat and send a message — verify it still appears at the top correctly
• Verify that resuming a thread without sending a message does not cause unnecessary reordering

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Summary by cubic

Fixes the chat history sidebar not moving the active thread to the top after sending a message in a resumed chat. The active conversation now reorders immediately.

• Bug Fixes
  • Detect new user messages by tracking messages.length in ThreadHistory.
  • Update the active thread’s createdAt to now, triggering the existing groupByDate sort.
  • No reordering on initial load or when resuming without sending a message.

^{Written for commit a20b60d. Summary will update on new commits.}

[cubic-dev-ai]

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