remove mobile header bar and replace with floating hamburger.
restyle mobile nav links with icons, active states, compact spacing.
integrate theme toggle into nav flow, remove redundant cta button.
- replace full-width header with floating top-right hamburger
- add icons to mobile nav links (matching desktop)
- reduce text size (text-3xl -> text-base) and spacing (gap-8 -> gap-3)
- implement active state indicators using isActive()
- add hover states with proper theme variables
- move theme toggle from decorative area into main nav with divider
- remove "let's talk" cta button (redundant with contact link)