fix: polish mobile responsive layout

[kingzzoov-ctrl]

Closes #833

Summary

• tighten the hero terminal card for 375px mobile screens so the typewriter line stays within the viewport
• make the live stats wrap instead of forcing horizontal overflow
• improve the mobile navigation menu with route-close behavior, accessible hamburger attributes, and a scroll-bounded dropdown
• adjust bounty cards/grid spacing so cards stack cleanly on mobile and use the wider tablet breakpoint before moving to three columns

Verification

• \ passes
• \ currently fails on the existing frontend baseline because shared modules such as \ and \ are missing before this PR's changes
• \Unknown command: "test"

Did you mean this?
npm test # Test a package

To see a list of supported npm commands, run:
npm help currently fails on the existing frontend baseline because multiple tests import missing components/hooks and Playwright is not installed; one unrelated API client suite still passes