Polish mobile responsive layout

[Vinzz2303]

Closes #833

Summary

• polish bounty cards so reward/meta/status rows wrap cleanly at narrow mobile widths
• improve hero readability on small screens with responsive terminal, heading, CTA, and stats-strip sizing
• harden mobile navigation behavior with a proper hamburger flow, route-close behavior, and body scroll locking while the menu is open
• restore the missing frontend animation/util helpers so the responsive frontend branch builds cleanly again

Scope

This PR is focused on the responsive polish requested in #833:

• bounty cards
• navigation / hamburger behavior
• hero readability
• preventing horizontal overflow at the tested mobile/tablet breakpoints

Validation

• npm install
• npm run build
• manual responsive verification at 375px and 768px

Manual Checks

• homepage looks correct at 375px and 768px
• bounty list page looks correct at 375px and 768px
• mobile nav switches to hamburger mode and no horizontal scroll was observed in the checked views

Files

• frontend/src/components/bounty/BountyCard.tsx
• frontend/src/components/home/HeroSection.tsx
• frontend/src/components/layout/Navbar.tsx
• frontend/src/lib/animations.ts
• frontend/src/lib/utils.ts

[Vinzz2303]

Validation evidence for this mobile-responsive polish:\n\n-
pm install\n-
pm run build\n- manual responsive verification at 375px and 768px on the homepage, How It Works, and /bounties\n\nWhat was checked:\n- bounty cards stack and wrap cleanly on narrow screens\n- hamburger/mobile navigation behavior is present\n- hero/terminal content remains readable on small screens\n- no horizontal overflow was observed at the requested breakpoints\n\nThis PR intentionally covers the full issue scope across cards, navigation, hero readability, and overflow polish rather than only a subset of the mobile surfaces.