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.

assets/css/admin.css

@@ -0,0 +1,19 @@
+.siti-stock-settings {
+  max-width: 800px;
+}
+
+.siti-stock-settings .description {
+  color: #555;
+}
+
+.siti-stock-cron-info {
+  margin-top: 20px;
+  border-left: 4px solid #2271b1;
+  background: #fff;
+  padding: 16px;
+  display: none;
+}
+
+.siti-stock-cron-info.is-visible {
+  display: block;
+}

assets/js/admin.js

@@ -0,0 +1,15 @@
+(function ($) {
+  function toggleCronInfo() {
+    var info = $('.siti-stock-cron-info');
+    var checkbox = $('[data-siti-stock-toggle="cron"]');
+
+    if (!info.length || !checkbox.length) {
+      return;
+    }
+
+    info.toggleClass('is-visible', checkbox.is(':checked'));
+  }
+
+  $(document).on('change', '[data-siti-stock-toggle="cron"]', toggleCronInfo);
+  $(toggleCronInfo);
+})(jQuery);