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/components/App.js

@@ -0,0 +1,39 @@
+/**
+ * App Component - Main admin application shell
+ */
+
+import OrdersPage from '../pages/Orders';
+import CustomersPage from '../pages/Customers';
+import ProductsPage from '../pages/Products';
+import FormsPage from '../pages/Forms';
+import CouponsPage from '../pages/Coupons';
+import AccessPage from '../pages/Access';
+import LicensesPage from '../pages/Licenses';
+
+const pageComponents = {
+    orders: OrdersPage,
+    customers: CustomersPage,
+    products: ProductsPage,
+    forms: FormsPage,
+    coupons: CouponsPage,
+    access: AccessPage,
+    licenses: LicensesPage,
+};
+
+export default function App({ page, initialData }) {
+    const PageComponent = pageComponents[page];
+
+    if (!PageComponent) {
+        return (
+            <div className="formipay-error">
+                <p>Unknown page: {page}</p>
+            </div>
+        );
+    }
+
+    return (
+        <div className="formipay-admin-wrap">
+            <PageComponent initialData={initialData} />
+        </div>
+    );
+}