Replace scrollToIndex with built-in followOutput (#58)

* Replace scrollToIndex with built-in followOutput

- Remove manual scrollToIndex implementation in favor of Virtuoso's
  followOutput prop
- Set followOutput to a function that always returns true for consistent
  scrolling during fast updates
- Adjust atBottomThreshold from 100 to 6 to fix messages being cut off
- Add padding-bottom: 6px to .message class for proper scroll alignment
- Fixes issue where messages appear halfway cut off at the bottom

* Use smooth scrolling for followOutput

Author: BP602