Cover layout does not fully cover bottom area / leaves unused gap

### Checklist

- [x] Using latest version on the [App Store](http://itunes.apple.com/app/id1156707581)
- [x] Read the [docs](http://blink.sh/docs/)
- [x] Searched for [existing GitHub issues](https://github.com/blinksh/blink/issues)

### Configuration

Blink v18.4.2.1052
iPadOS 26.1
iPad Pro M5 13-inch

### Describe the bug

Regardless of font-size, an area at the bottom of the screen is not covered by 'Cover' layout. Font size can affect the apparent height of the gap but I believe there is always a minimum height to the gap for which is never possible to cover.

Note, it would make sense for there to be an up-to-a-line-height gap to avoid characters being clipped off the bottom of the screen. Similarly, I would expect the right-side (if left-to-right locale) to have an up-to-one-character-width gap to avoid truncation (Blink appears to do this correctly). However, there is always a set gap at the bottom of the screen regardless of what font/size is used.

tmux status bar is raised more than the font line height from the bottom of the screen because of an un-coverable gap.

Reproduce the gap on Blink v18.4.2.1052 with an iPad Pro M5 13-inch running iPadOS 26.1, varying the font size and using the Cover layout. Trace the layout’s bottom inset or sizing behavior and verify that the remaining gap is no larger than the intended line-height safety margin without clipping terminal content.

Written by the indexing model from the issue text.