fixing auth server

frontend/assets/js/app.js

@@ -61,7 +61,13 @@ async function performLogin(authToken) {
             headers: { 'Content-Type': 'application/json' },
             body: JSON.stringify({ auth_token: authToken })
         });
-        const data = await res.json();
+        const text = await res.text();
+        let data;
+        try { data = JSON.parse(text); } catch (e) {
+            errEl.textContent = 'Server returned: ' + text.substring(0, 100);
+            errEl.style.display = 'block';
+            return;
+        }
         if (data.status === 'success') {
             currentUser = data.username;
             currentRole = data.role;