Show task length in recent task history

[wpbonelli]

Currently the recent task history plot on the home page shows each task as a point (its creation time), e.g.

A Gantt chart showing task duration would be better. (If/when we support task composition, we can also track dependencies this way.) This looks like a good solution to work from.

[wpbonelli]

This is easier to translate to get an initial impl working since it's not Observable

[wpbonelli]

Would also be nice to have the timeline on the task view update in real time after status checks (right now it only updates twice, first on creation then on completion)