Screen Share PiP Placement: Snapping positions & tile view

### Your use case

#### What would you like to do?
Allow the screen share PiP to be able to sit on top the left and right panels
#### Why would you like to do it?
Have a clear area for viewing call participants.
#### How would you like to achieve it?
Allow the PiP screen share to be moveable across the entire screen and snap to corners.

### Have you considered any alternatives?
Have the preview show up in its own tiled window, like a participant.

### Additional context
Currently the screen share PiP view goes behind the left and right panels limiting space/obstructing view.

Start by tracing the screen-share PiP placement and the left and right panel layout in the frontend, then reproduce the current overlap. The issue does not name files or tests; clarify whether the intended result is movable corner-snapping PiP, a tiled participant-style window, or both, and consider the work done when the chosen behavior no longer obstructs participant viewing.

Written by the indexing model from the issue text.