ui: tabs should count as >1 column when moving a text cursor up/down

When moving the cursor up and down with the directional pad, we track the column of the cursor and move it that column in the new line (or eol if the line has fewer columns). When counting columns, tabs currently count as 1 column. They should count as >1 (probably either 4 or 8 — try other textbox implementations and see what they do, and see what feels natural).

This is minor, but using 1 column for tab ends up feeling a bit awkward, because you expect the cursor to move only very little in the horizontal direction when hitting up/down (it's proportional font, so it won't move exactly vertical, but shouldn't move _too_ much horizontally). However, with tabs counting as 1 column but drawing as 8 spaces, the cursor can move a lot horizontally in the face of tabs.

Locate the cursor movement logic that tracks columns for up/down movement and the code that renders tabs. Compare how other textbox implementations treat tab width, then choose a behavior that keeps vertical movement natural. Done means tabs count as more than one column and up/down movement no longer jumps excessively across lines containing tabs.

Written by the indexing model from the issue text.