Fix sitemap

Most importantly, make it work globally.

load.lib.php

@@ -1,9 +1,9 @@
 <?php
 
-if ($included !== true) {
-    header('HTTP/1.0 403 Forbidden');
-    header('Location: /en/error/403');
-    exit;
+function isHttps(): bool {
+    // https://www.designcise.com/web/tutorial/how-to-check-for-https-request-in-php
+    $isHttps = $_SERVER['HTTPS'] ?? $_SERVER['REQUEST_SCHEME'] ?? $_SERVER['HTTP_X_FORWARDED_PROTO'] ?? null;
+    return $isHttps && (strcasecmp('on', $isHttps) === 0 || strcasecmp('https', $isHttps) === 0);
 }
 
 function readPage(string $page): bool {