feat: initialize React admin build pipeline (F2.1-F2.6)

- Add package.json with @wordpress/scripts and React dependencies
- Configure webpack for admin bundle output
- Create src/admin directory structure (api, components, pages)
- Implement API client with nonce handling (ajaxRequest, apiRequest)
- Add API methods for orders, customers, products, forms, coupons, licenses
- Create React App component with page routing
- Add placeholder page components for all admin sections
- Create ReactAdmin PHP class to manage asset enqueuing
- Register ReactAdmin singleton in main plugin file
- Bump version to 2.0.0

Build: Run 'npm install && npm run build' to generate assets

src/admin/pages/Coupons.js

@@ -0,0 +1,14 @@
+/**
+ * Coupons Page - Placeholder
+ */
+
+import { __ } from '@wordpress/i18n';
+
+export default function CouponsPage({ initialData }) {
+    return (
+        <div className="formipay-page-formipay-page">
+            <h1>{ __('Coupons', 'formipay') }</h1>
+            <p>{ __('Page content coming soon...', 'formipay') }</p>
+        </div>
+    );
+}