Housekeeping: offene Dev-Dateien eingecheckt + Sidebar „Bestellungen"→„Versand"

- Sidebar-Menü „Bestellungen" in „Versand" umbenannt (layout-sidenav), sonst nur
  Blade-Formatierung.
- Entwicklungspläne in ISO-Datumsformat umbenannt (entwicklungsplan-aktualisiert-*).
- dev/dhl-for-woocommerce/ (DHL-Plugin) + dev/partnercenter-api.zip + PastedGraphic
  hinzugefügt.
- .claude/ bewusst NICHT eingecheckt (lokale Claude-Code-Konfiguration).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

dev/dhl-for-woocommerce/includes/class-pr-dhl-libraryloader.php

@@ -0,0 +1,91 @@
+<?php
+
+use PR_DHL\lib\PDFMerger\PDFMerger;
+
+if (! defined('ABSPATH')) {
+    exit;
+}
+
+if (! class_exists('PR_DHL_Libraryloader')) {
+
+    class PR_DHL_Libraryloader
+    {
+        const CLASS_PDF_MERGER = 'pdf_merger';
+
+        private static $instances = [];
+
+        protected $include_path = null;
+
+        protected $file_path = null;
+
+        protected $loaded = [];
+
+        /**
+         * Constructor.
+         */
+        public function __construct()
+        {
+            // Set paths
+            $this->include_path = PR_DHL_PLUGIN_DIR_PATH.DIRECTORY_SEPARATOR.'lib'.DIRECTORY_SEPARATOR;
+            $this->file_path = $this->include_path.'PDFMerger'.DIRECTORY_SEPARATOR.'PDFMerger.php';
+            $this->file_path = str_replace(['/', '\\'], DIRECTORY_SEPARATOR, $this->file_path);
+        }
+
+        /**
+         * Returns a singleton instance of the called class
+         *
+         * @return static
+         */
+        public static function instance()
+        {
+
+            $class = get_called_class();
+            if (! isset(self::$instances[$class])) {
+                self::$instances[$class] = new static;
+            }
+
+            return self::$instances[$class];
+
+        }
+
+        public function get_pdf_merger()
+        {
+            if (! class_exists('PR_DHL\lib\PDFMerger\PDFMerger')) {
+                $loaded = $this->include_file($this->file_path);
+
+                if (! $loaded) {
+                    return null;
+                }
+                $this->loaded[] = self::CLASS_PDF_MERGER;
+            }
+
+            // Something very unexpected happened, return
+            if (! class_exists('PR_DHL\lib\PDFMerger\PDFMerger')) {
+                return null;
+            }
+
+            return new PDFMerger;
+        }
+
+        /**
+         * Include a file if it exists.
+         *
+         * @return bool
+         */
+        protected function include_file($path)
+        {
+
+            if (file_exists($path)) {
+                // Supress errors of third party libraries
+                $status = @include_once $path;
+                if ($status !== 1) {
+                    return false;
+                }
+
+                return true;
+            }
+
+            return false;
+        }
+    }
+}