fix(desktop): use the round brand logo for the default Agent avatar

The default Agent face used the wide-margin square logo.png scaled to 72%,
so it looked small and, once cropped into the circle, read as an octagon.
Use the same round logo.jpg the login/chat screens and web console use,
filled edge-to-edge, so circle faces are clean discs and chat bubbles fill.

Co-authored-by: cowagent <cow@cowagent.ai>