At mobile widths the resizable git panel collided with the chat column's
360px min-width, overflowing the viewport. Treat mobile (<768px) like the
sidebar: the git panel becomes a full-screen overlay the user navigates to
and back from, while desktop keeps the inline resizable panel.
Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
