Mobile activity feed: merge columns into single chronological list

[robzolkos]

On desktop the Added/Updated/Done columns work well - you can scan all three at a glance. On mobile they stack vertically, which means events appear grouped by type rather than by time. A card closed at 3pm shows up below cards added at 9am, because Done renders after Added and Updated.

Current:

On a single-column screen, a reverse-chronological list of all activity would be more natural - the most recent thing that happened appears first regardless of whether it was an add, update, or close.

Proposed:

As a user who frequently checks activity on my mobile it is pretty annoying, so happy to take a crack at fixing this.
(I checked all open/closed issues and discussions and found no mentions of this but it may have been considered elsewhere or a plan already in place)

cc @jzimdars