Implement pure CSS mobile navigation with the following features:
- Collapsible sidebar nav hidden behind hamburger toggle on mobile
- Animated hamburger → X icon morph using CSS transforms
- Smooth slide-down transition for nav reveal (max-height/opacity)
- Hamburger positioned on right side of header via absolute positioning
- Uses :has() selector to detect checkbox state without JavaScript
- Overflow fixes to prevent horizontal scroll on mobile devices
The implementation follows suckless principles with zero JavaScript.
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