prompt before closing tab mid-upload

static/js/upload.js

@@ -29,6 +29,12 @@ let progressRate;
 let progressEta;
 let progressBar;
 
+window.addEventListener('beforeunload', (event) => {
+    if (socket && socket.readyState !== 3) {
+        event.returnValue = "Upload is still in progress!";
+    }
+});
+
 document.addEventListener('DOMContentLoaded', () => {
     document.body.className = 'landing';