feat: upscale landing page text sizes and spacing

Bump all small text from hardcoded px values to standard Tailwind sizes:
text-[7-8px] → text-[9-10px], text-[9-12px] → text-xs, text-[13px] → text-sm,
text-[14px] → text-sm, text-[15px] → text-base. Increase hero heading by
one step per breakpoint, bump header nav text and padding, and widen
content container from max-w-4xl to 60rem.