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.

includes/class-siti-stock-admin-notices.php

@@ -0,0 +1,58 @@
+<?php
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+/**
+ * Handles temporary admin notices.
+ */
+class Siti_Stock_Admin_Notices {
+
+	const TRANSIENT_KEY = 'siti_stock_notice';
+
+	/**
+	 * Register hooks.
+	 */
+	public function register_hooks() {
+		add_action( 'admin_notices', array( $this, 'render_flash_notice' ) );
+	}
+
+	/**
+	 * Store notice in transient for later output.
+	 *
+	 * @param string $message Message.
+	 * @param string $type Notice type.
+	 */
+	public function add_notice( $message, $type = 'success' ) {
+		set_transient(
+			self::TRANSIENT_KEY,
+			array(
+				'message' => $message,
+				'type'    => $type,
+			),
+			30
+		);
+	}
+
+	/**
+	 * Render notice if stored.
+	 */
+	public function render_flash_notice() {
+		$notice = get_transient( self::TRANSIENT_KEY );
+
+		if ( ! $notice ) {
+			return;
+		}
+
+		delete_transient( self::TRANSIENT_KEY );
+
+		$type = in_array( $notice['type'], array( 'error', 'warning', 'success', 'info' ), true ) ? $notice['type'] : 'info';
+
+		printf(
+			'<div class="notice notice-%1$s is-dismissible"><p>%2$s</p></div>',
+			esc_attr( $type ),
+			esc_html( $notice['message'] )
+		);
+	}
+}