feat: improve sandbox footer UI readability and mobile responsiveness

[nitin-999-code]

Improves the footer UI on the Axios sandbox for better readability and user experience.

Changes Made

• Increased footer font size from 0.9em to 1em for better readability
• Enhanced navigation links with improved hover effects and padding
• Added smooth transitions and subtle transform effects on hover
• Improved mobile responsiveness with vertical navigation layout
• Added button-like styling for mobile links with proper spacing
• Removed underline on hover for cleaner appearance
• Maintained consistent design aesthetic with existing theme

Issues Addressed

• Footer font size was too small compared to other text elements
• Links could be missed due to small sizing
• Mobile responsiveness needed improvement