feat: mobile responsive polish for bounty cards, nav, hero, and pages [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Summary

Mobile responsive polish for the SolFoundry frontend, addressing issue #833.

Changes

HeroSection

• Responsive headline: text-2xl on mobile (was text-4xl)
• Terminal card: smaller padding on mobile, side margins
• Subtitle: responsive font size
• Stats strip: wraps on narrow screens

BountyCard

• Smaller padding on mobile (p-4 vs p-5)
• Status badge moved from absolute positioning to inline flow
• Reward amount truncates to prevent overflow

BountyGrid

• Filter pills scroll horizontally on mobile (hidden scrollbar)

Navbar

• Larger hamburger touch target (p-2.5)
• Added aria-label for accessibility

BountyDetail

• Title: text-xl on mobile (was text-2xl)
• Reward: text-2xl on mobile (was text-3xl)

ProfileDashboard

• Responsive font sizes
• Scrollable tab switcher on mobile

Pages (Leaderboard, How It Works, Post a Bounty)

• Responsive page titles (text-3xl on mobile, text-4xl on desktop)

Global

• html { overflow-x: hidden } prevents horizontal scroll
• .scrollbar-none utility for hiding scrollbars on scrollable containers

Closes #833