R1:be1e3b2cc0a8

When a user drags a card over a column, highlight the column border

Summary:
Ref T10334. Partly, this just improves visual feedback for all drag operations. After D20242, we can have cases where you (for example) drag a low-priority node to a very tall column on a priority-ordered workboard. In this case, the actual dashed-border-drop-target may not be on screen.

We might make the column scroll or put some kind of hint in the UI in this case, but an easy starting point is just to make the "yes, you're…