Scroll off-by-one when pressing Return at bottom of viewport

> When you're at the bottom of the screen writing and press return, the window needs to scroll 1 line. Feels off by one. Scroll happens after typing first char, which is helpful but it should happen right before then.

## Claude Opus 4.6's Interpretation

When typing at the last visible line and pressing Return, the new line is below the viewport. Scrolling only happens after typing the first character on that line. The scroll should happen immediately when Return creates a new line at or past the viewport bottom.

### Possible solution

- Hook into the Return key or `on_text_area_changed` to check if cursor is below viewport and scroll proactively