Minor CSS Fix extra spacing for collapsed empty groups in left sidebar

[jeffscottward]

Bug

When a group folder in the left sidebar is collapsed and contains no child agents, the UI still renders a collapsed group indicator placeholder area.

Expected

Collapsed empty groups should not reserve vertical space below the group row for missing agent bars.

Actual

There is always a small blank gap/palette area when collapsed, even with no agents.

Impact

Visual layout gets extra vertical whitespace and looks inconsistent between empty and populated groups.