Notebook widget should dynamically size to the width of the notebook

[jleibs]

Here's an example of a super irritating situation:

1751323026.mp4

My content would otherwise fit just fine, but the viewer is being arbitrarily scaled to a fixed with which was too small for my screen and now I have to scroll.

[jprochazk]

A good starting point would be to add a width=AUTO mode, where the viewer would take up 100% of the output cell's width. We'd also need to handle height properly. If width=AUTO and height is not set explicitly, then height=AUTO as well. height=AUTO doesn't mean it takes up 100% of anything, it just adjusts the height to scale with the width according to some reasonable aspect ratio (e.g. 4:3 or 16:9)