security: extract iCloud credentials into protected config file

- Removed hardcoded iCloud username and password from Data.class.php
- Added config/icloud.php for external credential storage
- Protected config directory with Apache access rules
- Updated Data constructor to load credentials from config
- Kept backwards compatibility with existing Data instantiation

config/icloud.php

@@ -0,0 +1,13 @@
+<?php
+/**
+ * iCloud CalDAV Konfiguration
+ * 
+ * Diese Datei enthält die Anmeldedaten für den iCloud-Kalender-Sync.
+ * Geschützt durch .htaccess (Deny from all).
+ */
+return [
+    'calendar_url' => 'https://p144-caldav.icloud.com/18998614209/calendars/ea9819c0-518a-4f23-ba11-476f639fd66e/',
+    'username'     => 'itzanjuma@gmail.com',
+    'password'     => 'ywce-toxv-whvv-iorx',
+];
+