This PR simplifies the flexible-width agent panel's rendering such that
for now, it will not grow to fill the center area when there are no
files open. The problem with doing that is that it makes the bottom dock
invisible.

We could potentially revisit this, and just shrink the width of the
agent panel when the bottom dock was toggled, but that might feel
unstable, so for now I'm doing a simpler thing and just keeping the
center area visible even when it is empty.

Release Notes:

- N/A