error in login

backend/login.php

@@ -1,7 +1,6 @@
 <?php
 session_start();
 
-// If already logged in, redirect
 if (isset($_SESSION['neptune_loggedin']) && $_SESSION['neptune_loggedin'] === true) {
     header('Location: /');
     exit;
@@ -10,7 +9,11 @@ if (isset($_SESSION['neptune_loggedin']) && $_SESSION['neptune_loggedin'] === tr
 $error = '';
 $success = '';
 
-// Check for auth callback from Jakach Auth
+// Detect the correct callback URL
+$scheme = (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off') ? 'https' : 'http';
+$host = $_SERVER['HTTP_HOST'] ?? 'localhost:8080';
+$callbackUrl = "$scheme://$host/login.php";
+
 if (isset($_GET['auth'])) {
     $auth_token = $_GET['auth'];
     $check_url = "https://auth.jakach.ch/api/auth/check_auth_key.php?auth_token=" . urlencode($auth_token);
@@ -98,7 +101,7 @@ if (isset($_GET['auth'])) {
             <div class="alert alert-success py-2 small"><?= htmlspecialchars($success) ?></div>
         <?php endif; ?>
 
-        <a href="https://auth.jakach.ch/?send_to=<?= urlencode('http://' . ($_SERVER['HTTP_HOST'] ?? 'localhost:8080') . '/login.php') ?>" class="btn btn-jakach w-100 py-2 mb-2">
+        <a href="https://auth.jakach.ch/?send_to=<?= urlencode($callbackUrl) ?>" class="btn btn-jakach w-100 py-2 mb-2">
             <i class="fas fa-right-to-bracket me-2"></i>Log in with Jakach Auth
         </a>
         <small class="text-secondary">First user automatically becomes admin</small>