Fix sizeToFit() not including gutter width in frame calculation (#101)

## Problem
When `isHorizontallyResizable=true` and `showsLineNumbers=true`, I'm
unable to scroll all the way to the right of the view.

### Gutters enabled (bug)
<img width="1470" height="915" alt="Screenshot 2026-01-05 at 6 28 07 PM"
src="https://github.com/user-attachments/assets/9a25e843-6955-4d2d-a08e-88a79e28a4b8"
/>

### Gutters disabled (no bug!)
<img width="1470" height="915" alt="Screenshot 2026-01-05 at 6 27 19 PM"
src="https://github.com/user-attachments/assets/d28dd917-945f-4f9b-a711-ff3e5926fcb8"
/>

## Hypothesis
I think `sizeToFit()` is not adding the gutter width (when
`intrinsicContentSize` & `updateContentSizeIfNeeded` are?) which leads
to a mismatch & thus the clipping.

I unfortunately cannot test this because of other build issues – but
maybe enough info to head in the right direction?
<img width="773" height="200" alt="Screenshot 2026-01-05 at 7 05 21 PM"
src="https://github.com/user-attachments/assets/7464e6a1-8921-4378-8612-756089378e37"
/>