feat: Implement Siti Stock Plugin for WooCommerce integration

- Added core plugin structure with main class Siti_Stock_Plugin.
- Implemented settings management through Siti_Stock_Settings.
- Developed admin interface for settings configuration via Siti_Stock_Admin.
- Created inventory management with external stock handling in Siti_Stock_Inventory_Manager.
- Integrated synchronization service to fetch and apply stock updates from external API in Siti_Stock_Sync_Service.
- Added custom product data store to manage combined stock values in Siti_Stock_Product_Data_Store.
- Registered hooks for admin menus, settings, and synchronization processes.
- Implemented REST API endpoint for triggering stock sync.
- Added cron scheduling for automatic stock synchronization.
- Included localization support for Dutch language.

siti-stock-plugin.php

@@ -0,0 +1,50 @@
+<?php
+/**
+ * Plugin Name:       Siti Stock Plugin
+ * Plugin URI:        https://github.com/SitiWeb/siti-stock-plugin
+ * Description:       Synchroniseert WooCommerce voorraad met het externe Siti voorraadplatform.
+ * Version:           0.1.0
+ * Author:            Siti Web
+ * Author URI:        https://www.siti.nl
+ * Requires PHP:      8.1
+ * Requires at least: 6.4
+ * Text Domain:       siti-stock-plugin
+ * Domain Path:       /languages
+ */
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+define( 'SITI_STOCK_PLUGIN_VERSION', '0.1.0' );
+define( 'SITI_STOCK_PLUGIN_FILE', __FILE__ );
+define( 'SITI_STOCK_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );
+
+require_once __DIR__ . '/includes/class-siti-stock-plugin.php';
+require_once __DIR__ . '/SitiWebUpdater.php';
+
+register_activation_hook( __FILE__, array( 'Siti_Stock_Plugin', 'activate' ) );
+register_deactivation_hook( __FILE__, array( 'Siti_Stock_Plugin', 'deactivate' ) );
+
+add_action(
+	'init',
+	function () {
+		load_plugin_textdomain( 'siti-stock-plugin', false, dirname( plugin_basename( __FILE__ ) ) . '/languages' );
+	}
+);
+
+Siti_Stock_Plugin::instance();
+
+add_action(
+	'admin_init',
+	function () {
+		if ( ! class_exists( 'SitiWebUpdater' ) ) {
+			return;
+		}
+
+		$updater = new SitiWebUpdater( __FILE__ );
+		$updater->set_username( 'SitiWeb' );
+		$updater->set_repository( 'siti-stock-plugin' );
+		$updater->initialize();
+	}
+);