fix(BUG-025): attach click listener to copy button on API key display

public/app.js

@@ -145,6 +145,7 @@ document.addEventListener('DOMContentLoaded', function() {
   document.getElementById('btn-close-signup').addEventListener('click', closeSignup);
   document.getElementById('signupBtn').addEventListener('click', submitSignup);
   document.getElementById('verifyBtn').addEventListener('click', submitVerify);
+  document.getElementById('copyBtn').addEventListener('click', copyKey);
   document.getElementById('signupModal').addEventListener('click', function(e) {
     if (e.target === this) closeSignup();
   });