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/index.js

@@ -0,0 +1,28 @@
+/**
+ * Formipay Admin - React Application Entry Point
+ */
+
+import { render } from '@wordpress/element';
+import App from './components/App';
+
+// Mount the React app to all available mount points
+const mountApps = () => {
+    const mountPoints = document.querySelectorAll('[data-formipay-mount]');
+
+    mountPoints.forEach((mountPoint) => {
+        const page = mountPoint.dataset.formipayMount;
+        const initialData = window.formipayAdmin?.[page] || {};
+
+        render(
+            <App page={page} initialData={initialData} />,
+            mountPoint
+        );
+    });
+};
+
+// Initialize when DOM is ready
+if (document.readyState === 'loading') {
+    document.addEventListener('DOMContentLoaded', mountApps);
+} else {
+    mountApps();
+}