fix: correct login button selector in auth.js

Changed querySelector('.login-btn') to getElementById('login-btn') to match index.html.

static/js/auth.js

@@ -58,7 +58,7 @@ class AuthManager {
 
     async login(username, password) {
         const errorElement = document.getElementById('login-error');
-        const loginBtn = document.querySelector('.login-btn');
+        const loginBtn = document.getElementById('login-btn');
         
         try {
             loginBtn.innerHTML = '<i class="fas fa-spinner fa-spin"></i> Authenticating...';