Sidebar expand-to-fullwidth feature

[nate-layman]

Problem

The sidebar currently has only two states (closed/open at 40%). We want to add a three-state system:

1. Closed: Native bslib < toggle (top-right)
2. Open (40%): Show expand button to go fullwidth
3. Expanded (100%): Show collapse button to return to 40%

Challenge

bslib's native toggle only supports open/close. Adding a third "expand" state requires:

• Either hiding the native toggle and building a custom system (complex)
• Or keeping the native toggle and carefully coordinating an expand button (needs deduplication)

Current Attempt Issues

• Duplicate close buttons (native + custom)
• Need to hide native toggle or coordinate with it cleanly

Next Steps

1. Decide: Hide native toggle completely and rebuild, OR keep native toggle and add only expand?
2. Implement chosen approach
3. Test state transitions