feat(03-01): create settings class with WordPress Settings API

- Settings class with singleton pattern
- Registered 5 settings: receiver_email, captcha_provider, captcha_site_key, captcha_secret_key, thankyou_url
- Proper sanitization callbacks for each setting type
- Helper method get_option() for clean API access
- Integrated with admin menu structure

umzugsliste.php

@@ -52,6 +52,7 @@ class Umzugsliste {
 	private function load_dependencies() {
 		require_once UMZUGSLISTE_PLUGIN_DIR . 'includes/class-cpt.php';
 		require_once UMZUGSLISTE_PLUGIN_DIR . 'includes/class-admin-menu.php';
+		require_once UMZUGSLISTE_PLUGIN_DIR . 'includes/class-settings.php';
 	}
 
 	/**
@@ -73,6 +74,9 @@ class Umzugsliste {
 		if ( is_admin() ) {
 			Umzugsliste_Admin_Menu::get_instance();
 		}
+
+		// Initialize settings
+		Umzugsliste_Settings::get_instance();
 	}
 }