bug: context picker selection disappears in compact terminal

[nathanhuh]

Summary

In the context picker, the selected context row can disappear or appear clipped when the terminal height is small.

Reproduction

1. Run unic and open the context picker.
2. Resize the terminal to a compact height, around 10-14 rows.
3. Move down through enough contexts for the table to scroll.

Expected

The selected context row remains visible and highlighted while navigating.

Actual

The selected row can disappear from the visible table area or the view can be middle-truncated.

Notes

This appears specific to the Bubbletea table-backed context picker rather than the regular list views.