🐛 fix(learning-area): stabilize expand button positioning and content transition in site shell

[b-l-i-n-d]

• Smoothly transition the learning area content when expanding or collapsing the sidebar in site shell mode
• Anchor the expand/collapse button position at bottom-start without layout shift across collapsed and expanded states
• Prevent the expand/collapse button from overlapping the theme footer in site shell mode
• Ensure the learning area header maintains stacking precedence above the sidebar on desktop screens
• Provide bottom clearance in the sidebar so scrolling curriculum items are not obscured by the pinned button