style(css): increase hero h1 font size on desktop and mobile

Grow the hero heading clamp range (4rem->4.5rem min, 7.5rem->8.5rem max on
desktop; 3.2rem->3.6rem min, 4.8rem->5.2rem max on mobile) for a bolder
visual impact in the redesigned homepage.

Co-Authored-By: Claude <[email protected]>