Screenshare on Stage should adjust to Screen Format

[dkgrieshammer]

Currently the Stage Modal Window is fixed size; it should show the whole shared desktop, current view cuts parts of the desktop; ideally it should also be resizable to have a proper preview of what is shown as sketched out in this penpot

https://design.penpot.app/#/view/041b11e0-4935-11ec-b61d-6194167bb409?page-id=ce41b090-4922-11ec-a354-794827c67032&section=interactions&index=0&share-id=013d94e0-8aa6-11ec-a2fe-9eafb5cb1e0f