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

webpack.config.js

@@ -0,0 +1,14 @@
+const defaultConfig = require('@wordpress/scripts/config/webpack.config');
+const path = require('path');
+
+module.exports = {
+    ...defaultConfig,
+    entry: {
+        'admin': './src/admin/index.js',
+    },
+    output: {
+        path: path.resolve(__dirname, 'build'),
+        filename: '[name].js',
+        chunkFilename: '[name].[contenthash].js',
+    },
+};