Hide sessions list in panel mode

[jeandat]

When the continue.dev extension is shown in the panel in vscode, you have the sessions list always visible taking a lot of space for no reason. That's really frustrating. Screen space is rare and should be optimized.

I don't want to use a sidebar for continue since i need horizontal space to read comfortably and i also don't want to have to resize my sidebar all the times depending on which extension i'm working with.

The sessions list is a tiny features that is occasionnaly used and it should not be always visible.

Please add a collapse button or a divider or something in that spirit.

Tested with release and pre-release version (1.1.76)