Refactor to modern JavaScript and add collapsible custom coin section

[RonHachmon]

Modern JavaScript improvements:

• Replace inline event handlers with addEventListener pattern
• Use arrow functions consistently throughout the code
• Add DOM elements caching for better performance
• Use template literals for string interpolation
• Use optional chaining (?.) for safer null checks
• Use ternary operators for cleaner conditional logic
• Destructure objects where appropriate
• Add clear separation of concerns with dedicated functions

UI improvements:

• Add toggle button to show/hide custom coin options
• Custom coin section now hidden by default with "collapsed" class
• Smooth transition animations when toggling section visibility
• Add aria-expanded attribute for better accessibility
• Responsive design updates for toggle button on mobile

The refactored code is more maintainable, follows modern best practices, and provides a better user experience with the collapsible custom options.