fix: adjust scroll detection threshold in chat component

[fdb02983rhy]

Summary

Fix #14639

The threshold is 300px now, sometimes users have to "fight" with the threshold for stopping the auto scrolling.
Removing the threshold can make it easier for the users to stop the scrolling.

Tip

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods