Changed header height to be explicitly set using CSS variable --header-height instead of being determined by padding
Set responsive header heights:
Default: 100px
Mobile (480px): 80px
Small Mobile (320px): 60px
Ensured scroll-margin-top on headings uses the same --header-height variable
Main content padding-top uses the same --header-height variable
These changes ensure consistent header heights across all screen sizes and proper scroll positioning for anchor links.
