title_bar.show_sign_in disables the sign in button, which is shown by
default and serves no purpose without a Zed account.
Panel layout: project, outline, collaboration and git panels dock to
the right; the agent panel docks left.
Icon theme changed from Zed (Default) to Material Icon Theme. Note
this requires the Material Icon Theme extension to be installed;
Zed falls back to the default icons if it is absent, so a fresh
machine linking these dotfiles will not error.