Fix the sidebar collapse button in the collapsed state

Summary

The sidebar collapse/expand control appears to stop working once the sidebar is collapsed.

Acceptance criteria

• The sidebar control remains visible and operable in both expanded and collapsed states.
• Activating the control while collapsed expands the sidebar.
• Activating it while expanded collapses the sidebar.
• Mouse, touch, and keyboard activation all work.
• The icon and accessible label reflect the action that will occur.
• Focus remains predictable after toggling.
• The behavior works for signed-in and logged-out sidebars at supported viewport sizes.
• Add regression coverage for both state transitions.