Previously, the chat auto-scrolled whenever messages changed, even when
not streaming. This caused unwanted scrolling when switching chats or
loading messages from the database.

Now auto-scroll only triggers when:
- Messages are actively streaming (isStreaming is true)
- User hasn't manually scrolled away
- User is near the bottom of the chat

Changes:
- Added isStreamingByIdAtom to track streaming state
- Modified auto-scroll useEffect to check isStreaming before scrolling
- Preserved streamCount effect for initial scroll on stream start

🤖 Generated with [Claude Code](https://claude.com/claude-code)
    
<!-- This is an auto-generated description by cubic. -->
---

## Summary by cubic
Fixes chat auto-scroll so it only runs during active streaming,
preventing jumps when switching chats or loading message history.

- **Bug Fixes**
- Added isStreamingByIdAtom and gated the auto-scroll effect by per-chat
streaming state (updated effect deps).
  - Preserved the initial scroll on stream start via streamCount.

<!-- End of auto-generated description by cubic. -->

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Limits chat auto-scroll to when a conversation is actively streaming
and the user is near the bottom.
> 
> - **Frontend**
> - **Chat auto-scroll behavior**: Only auto-scrolls during active
streaming.
> - Adds `isStreamingByIdAtom` and reads per-chat `isStreaming` state.
> - Updates auto-scroll `useEffect` to require `isStreaming` and adds it
to dependencies.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
6580601cd974c2c01ddffdef78ed0ddbb2b2fa8d. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
Co-authored-by: Claude <noreply@anthropic.com>