Mobile landing page polish: stacked lockup, service card, nav bg fix

- Add stacked MACHINE / PAYMENTS PROTOCOL lockup SVG for mobile (md:hidden)
  while keeping wide single-line lockup for desktop (hidden md:block)
- Responsive lockup width: min(648px, 88vw) — no fixed min-width
- Mobile service logos: tap opens bottom-aligned card with prompt, copy
  button, and view all services link; tap outside to dismiss
- Service logos wrap on mobile (3 + 2 layout via flex-wrap)
- Desktop tooltips hidden on mobile (hidden md:flex), hover only on desktop
- Fix mobile nav background: override Vocs vocs:max-md:bg-surface on :root
  so mobile header matches primary background
- Brighter mobile service card with shadow and primary border
- Left-aligned prompt text in mobile card
- Nav dropdown hover uses heading color instead of accent
- Variant toggle at very bottom (z-index 9999) with horizontal layout

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: achalvs