fix: mobile responsive polish for bounty cards, navigation, and hero section (Closes #833) [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Summary

Mobile responsive polish for the SolFoundry landing page and React frontend.

Changes

Landing page (index.html):

• Full-screen mobile navigation drawer (slides from top, covers entire screen)
• Hamburger icon changes to X when menu is open
• Mobile menu closes when clicking a nav link
• CTA button moves into the mobile nav drawer (hidden on desktop)
• New 375px breakpoint styles for:
  • Smaller hero heading (1.8rem)
  • Smaller stat numbers (2rem)
  • Section titles and CTA text adjusted
  • Token feature grid column span fix
• overflow-x: hidden on body prevents horizontal scroll
• -webkit-overflow-scrolling: touch for smooth iOS scrolling

React Frontend Components:

BountyCard:

• Reduced padding on mobile (p-4 sm:p-5)
• Smaller reward font size on mobile (text-base sm:text-lg)

HeroSection:

• Responsive terminal card with horizontal margins on mobile
• Smaller headline and subtitle on mobile
• Smaller terminal body text on mobile
• Live stats strip wraps on mobile with reduced gap

BountyGrid:

• Filter pills horizontally scrollable on mobile (hidden scrollbar)
• Reduced gap between sections on mobile
• Smaller section title on mobile

LeaderboardTable:

• Added horizontal scroll wrapper for mobile (overflow-x-auto)
• Minimum width 500px to prevent table column collapse

PodiumCards:

• Smaller card widths on mobile (min-w-[100px] sm:min-w-[140px])
• Smaller avatar sizes on mobile
• Reduced gap between cards on mobile

Footer:

• Responsive 2-column grid on medium screens
• Reduced padding on mobile

index.css:

• Added scrollbar-none utility class
• Added -webkit-text-size-adjust: 100% for iOS

Verification

• All pages tested at 375px (iPhone SE) and 768px (iPad) breakpoints
• No horizontal scroll on any page
• Bounty cards stack properly in single column on mobile
• Navigation hamburger menu works correctly with full-screen drawer
• Hero terminal section is readable on small screens

Closes #833