Fix z-index layering issues
- Move z-index from .header to .header-wrapper to avoid creating
stacking context that traps ProfileDropdown
- Add z-index: 50 to ProverbBanner to ensure it stays below header
- Header wrapper now at z-index: 100, higher than page content
This fixes:
- ProfileDropdown now renders above ProverbBanner
- ProfileDropdown renders above login form separator line
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>