Fix mobile menu horizontal overflow

[yanziwei]

Summary

• keep the site footer from widening mobile pages beyond the viewport
• include the drawer border inside the fixed drawer width so the panel stays within the viewport

Fixes #46.

Testing

• npm run build
• git diff --check
• local mobile viewport check at /bounties/: before opening the menu, scrollWidth == clientWidth; after tapping Menu, the drawer wrapper is fully within the 390px viewport and nav links are visible
• checked browser console for runtime errors: none