phsl / new-api

- Convert copy button to Input suffix for cleaner UI design
- Add responsive grid layout for balance cards and preset amounts
  - Mobile (< md): single column layout for better readability
  - Desktop (>= md): multi-column layout for space efficiency
- Implement bottom fixed payment panel on mobile devices
  - Fixed positioning for easy access to payment options
  - Includes custom amount input and payment method buttons
  - Auto-hide on desktop to maintain original layout
- Improve mobile payment flow with sticky bottom controls
- Add proper spacing to prevent content overlap with fixed elements
- Maintain consistent functionality across all breakpoints

This update significantly improves the mobile user experience by making
payment controls easily accessible without scrolling, while preserving
the desktop layout and functionality.