better side panel interactions on mobile

[wchristian]

On mobile, the button for opening the side panel is hard to find, since it resides on the right side of the screen, while the menu is on the left side.

A better implementation might be a floating bubble in the top left.

Similarly, after opening the side panel, and interacting with it, focus switch and the subtlety of the change in the panel button can make it hard to find it again, when trying to close the side panel.

It would be good to simply highlight it to indicate it now acts as a closer.