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

includes/class-admin-menu.php

@@ -86,13 +86,8 @@ class Umzugsliste_Admin_Menu {
 	 * Settings page callback
 	 */
 	public function settings_page() {
-		?>
-		<div class="wrap">
-			<h1>Umzugsliste Einstellungen</h1>
-			<div class="notice notice-info">
-				<p><?php esc_html_e( 'Einstellungen werden in Phase 3 implementiert.', 'umzugsliste' ); ?></p>
-			</div>
-		</div>
-		<?php
+		// Delegate to settings class
+		$settings = Umzugsliste_Settings::get_instance();
+		$settings->render_settings_page();
 	}
 }

includes/class-settings.php

@@ -0,0 +1,138 @@
+<?php
+/**
+ * Settings Management
+ */
+
+// Exit if accessed directly
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+/**
+ * Settings class
+ */
+class Umzugsliste_Settings {
+
+	/**
+	 * Single instance
+	 */
+	private static $instance = null;
+
+	/**
+	 * Get instance
+	 */
+	public static function get_instance() {
+		if ( null === self::$instance ) {
+			self::$instance = new self();
+		}
+		return self::$instance;
+	}
+
+	/**
+	 * Constructor
+	 */
+	private function __construct() {
+		add_action( 'admin_init', array( $this, 'register_settings' ) );
+	}
+
+	/**
+	 * Register settings with WordPress Settings API
+	 */
+	public function register_settings() {
+		// Register receiver email setting
+		register_setting(
+			'umzugsliste_settings',
+			'umzugsliste_receiver_email',
+			array(
+				'type'              => 'string',
+				'sanitize_callback' => 'sanitize_email',
+				'default'           => '',
+			)
+		);
+
+		// Register captcha provider setting
+		register_setting(
+			'umzugsliste_settings',
+			'umzugsliste_captcha_provider',
+			array(
+				'type'              => 'string',
+				'sanitize_callback' => 'sanitize_text_field',
+				'default'           => 'none',
+			)
+		);
+
+		// Register captcha site key setting
+		register_setting(
+			'umzugsliste_settings',
+			'umzugsliste_captcha_site_key',
+			array(
+				'type'              => 'string',
+				'sanitize_callback' => array( $this, 'sanitize_api_key' ),
+				'default'           => '',
+			)
+		);
+
+		// Register captcha secret key setting
+		register_setting(
+			'umzugsliste_settings',
+			'umzugsliste_captcha_secret_key',
+			array(
+				'type'              => 'string',
+				'sanitize_callback' => array( $this, 'sanitize_api_key' ),
+				'default'           => '',
+			)
+		);
+
+		// Register thank you URL setting
+		register_setting(
+			'umzugsliste_settings',
+			'umzugsliste_thankyou_url',
+			array(
+				'type'              => 'string',
+				'sanitize_callback' => 'esc_url_raw',
+				'default'           => home_url(),
+			)
+		);
+	}
+
+	/**
+	 * Sanitize API key (trim whitespace)
+	 */
+	public function sanitize_api_key( $value ) {
+		return sanitize_text_field( trim( $value ) );
+	}
+
+	/**
+	 * Render settings page
+	 */
+	public function render_settings_page() {
+		// Check user capabilities
+		if ( ! current_user_can( 'edit_posts' ) ) {
+			return;
+		}
+
+		?>
+		<div class="wrap">
+			<h1>Umzugsliste Einstellungen</h1>
+			<form method="post" action="options.php">
+				<?php
+				settings_fields( 'umzugsliste_settings' );
+				do_settings_sections( 'umzugsliste_settings' );
+				submit_button();
+				?>
+			</form>
+		</div>
+		<?php
+	}
+
+	/**
+	 * Get option helper method
+	 *
+	 * @param string $key     Option key (without umzugsliste_ prefix)
+	 * @param mixed  $default Default value
+	 * @return mixed Option value
+	 */
+	public static function get_option( $key, $default = '' ) {
+		return get_option( 'umzugsliste_' . $key, $default );
+	}
+}

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();
 	}
 }