feat(03-01): add settings save confirmation

- Added settings_errors() call to display WordPress admin notices
- get_option() helper method already implemented for clean API access
- Settings now show success message after save
- All settings validated and persisted via WordPress Settings API

includes/class-settings.php

@@ -272,6 +272,7 @@ class Umzugsliste_Settings {
 		?>
 		<div class="wrap">
 			<h1>Umzugsliste Einstellungen</h1>
+			<?php settings_errors(); ?>
 			<form method="post" action="options.php">
 				<?php
 				settings_fields( 'umzugsliste_settings' );