PWA install button added

app.js

@@ -32,6 +32,7 @@ const STRINGS = {
     pctCorrect: '{pct}% correct',
     congratulations: 'Congratulations!',
     pleaseSelect: 'Please select all options',
+    installApp: 'Install App',
     systemNames: {
       binary: 'Binary', octal: 'Octal', hex: 'Hexadecimal',
       ternary: 'Balanced Ternary', braille: 'Braille Decimal',
@@ -68,6 +69,7 @@ const STRINGS = {
     pctCorrect: '{pct}% \u043F\u0440\u0430\u0432\u0438\u043B\u044C\u043D\u044B\u0445',
     congratulations: '\u041F\u043E\u0437\u0434\u0440\u0430\u0432\u043B\u044F\u0435\u043C!',
     pleaseSelect: '\u041F\u043E\u0436\u0430\u043B\u0443\u0439\u0441\u0442\u0430, \u0432\u044B\u0431\u0435\u0440\u0438\u0442\u0435 \u0432\u0441\u0435 \u043F\u0430\u0440\u0430\u043C\u0435\u0442\u0440\u044B',
+    installApp: '\u0423\u0441\u0442\u0430\u043D\u043E\u0432\u0438\u0442\u044C \u043F\u0440\u0438\u043B\u043E\u0436\u0435\u043D\u0438\u0435',
     systemNames: {
       binary: '\u0414\u0432\u043E\u0438\u0447\u043D\u0430\u044F',
       octal: '\u0412\u043E\u0441\u044C\u043C\u0435\u0440\u0438\u0447\u043D\u0430\u044F',
@@ -667,6 +669,31 @@ langSelect.addEventListener('change', () => {
 
 applyTranslations();
 
+let deferredPrompt = null;
+
+window.addEventListener('beforeinstallprompt', (e) => {
+  e.preventDefault();
+  deferredPrompt = e;
+  document.getElementById('btn-install').classList.remove('hidden');
+});
+
+document.getElementById('btn-install').addEventListener('click', async () => {
+  if (!deferredPrompt) return;
+  deferredPrompt.prompt();
+  await deferredPrompt.userChoice;
+  deferredPrompt = null;
+  document.getElementById('btn-install').classList.add('hidden');
+});
+
+window.addEventListener('appinstalled', () => {
+  deferredPrompt = null;
+  document.getElementById('btn-install').classList.add('hidden');
+});
+
+if (window.matchMedia('(display-mode: standalone)').matches || navigator.standalone) {
+  document.getElementById('btn-install').classList.add('hidden');
+}
+
 function showSetupMessage(msg) {
   const containers = document.querySelectorAll('.setup-container');
   let el = document.getElementById('setup-message');