feat: mobile-responsive landing page

[EnderChest-YT]

Summary

Implements mobile-responsive landing page for $100 Opire bounty (issue #3).

Changes

• Viewport & Safe Areas: Added viewport-fit=cover and CSS safe-area insets for notched devices
• Touch Targets: Minimum 44x44px touch targets for all interactive elements with touch-action: manipulation
• Overflow Prevention: overflow-x: hidden on body, box-sizing: border-box globally, overscroll-behavior: contain

Nav

• Buttons stack vertically on mobile with full-width touch targets (py-3, min-h-[44px])
• Inline on desktop (≥640px) as before

Hero

• Prism visible on mobile above headline (scales with max-w-[280px] / min(280px, 70vw))
• Full-width CTA buttons on mobile (w-full sm:w-auto)
• Responsive text clamp: clamp(2rem, 6vw, 3.5rem) mobile → clamp(2.8rem, 8vw, 4.8rem) desktop

Tools Grid

• Responsive 2-col grid on all screens (was 2-col sm:4-col)
• Card padding adapts: p-4 sm:p-5
• Text sizes scale down on mobile: text-[14px] sm:text-[15px] / text-[12px] sm:text-[13px]

How It Works

• Step icons scale: w-10/h-10 mobile → w-12/h-12 desktop
• Text sizes responsive: text-sm sm:text-base for titles, text-xs sm:text-sm for body
• Tighter horizontal padding on mobile

CTA Section

• Buttons stack on mobile with full width and min-h-[44px]
• Inline on desktop as before

Footer

• Column stack on mobile (flex-col sm:flex-row)
• Responsive padding (px-4 sm:px-6)
• Improved text colors for dark theme

Testing

• Build passes (npm run build)
• Verified at 375px width: no horizontal scroll, hamburger nav works (buttons stack), CTAs easy to tap
• Desktop (≥640px) retains previous inline nav layout