feat(01-01): create admin menu structure

- Created Umzugsliste_Admin_Menu class with singleton pattern
- Top-level menu 'Umzugsliste' with dashicons-list-view icon
- Position 25 (below Comments, above Appearance)
- Capability: edit_posts (editors and above)
- Submenu 'Einträge' links to CPT list view
- Submenu 'Einstellungen' shows placeholder (Phase 3)
- Removed duplicate top-level menu item

includes/class-admin-menu.php

@@ -0,0 +1,98 @@
+<?php
+/**
+ * Admin Menu Structure
+ */
+
+// Exit if accessed directly
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+/**
+ * Admin menu class
+ */
+class Umzugsliste_Admin_Menu {
+
+	/**
+	 * 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_menu', array( $this, 'register_menu' ) );
+	}
+
+	/**
+	 * Register admin menu
+	 */
+	public function register_menu() {
+		// Add top-level menu
+		add_menu_page(
+			'Umzugsliste',                    // Page title
+			'Umzugsliste',                    // Menu title
+			'edit_posts',                     // Capability
+			'umzugsliste',                    // Menu slug
+			array( $this, 'entries_page' ),   // Callback
+			'dashicons-list-view',            // Icon
+			25                                // Position
+		);
+
+		// Add Einträge submenu (CPT list)
+		add_submenu_page(
+			'umzugsliste',                    // Parent slug
+			'Einträge',                       // Page title
+			'Einträge',                       // Menu title
+			'edit_posts',                     // Capability
+			'edit.php?post_type=umzugsliste_entry' // Menu slug (link to CPT)
+		);
+
+		// Add Einstellungen submenu
+		add_submenu_page(
+			'umzugsliste',                    // Parent slug
+			'Einstellungen',                  // Page title
+			'Einstellungen',                  // Menu title
+			'edit_posts',                     // Capability
+			'umzugsliste-settings',           // Menu slug
+			array( $this, 'settings_page' )   // Callback
+		);
+
+		// Remove duplicate top-level menu item
+		remove_submenu_page( 'umzugsliste', 'umzugsliste' );
+	}
+
+	/**
+	 * Entries page callback (redirect handled by WordPress to CPT list)
+	 */
+	public function entries_page() {
+		// This won't be called as we're using edit.php link
+		wp_redirect( admin_url( 'edit.php?post_type=umzugsliste_entry' ) );
+		exit;
+	}
+
+	/**
+	 * 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
+	}
+}