fix: prevent asset conflicts between React and Grid.js versions

Add coexistence checks to all enqueue methods to prevent loading
both React and Grid.js assets simultaneously.

Changes:
- ReactAdmin.php: Only enqueue React assets when ?react=1
- Init.php: Skip Grid.js when React active on admin pages
- Form.php, Coupon.php, Access.php: Restore classic assets when ?react=0
- Customer.php, Product.php, License.php: Add coexistence checks

Now the toggle between Classic and React versions works correctly.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

node_modules/read-pkg/index.js

@@ -0,0 +1,41 @@
+'use strict';
+const {promisify} = require('util');
+const fs = require('fs');
+const path = require('path');
+const parseJson = require('parse-json');
+
+const readFileAsync = promisify(fs.readFile);
+
+module.exports = async options => {
+	options = {
+		cwd: process.cwd(),
+		normalize: true,
+		...options
+	};
+
+	const filePath = path.resolve(options.cwd, 'package.json');
+	const json = parseJson(await readFileAsync(filePath, 'utf8'));
+
+	if (options.normalize) {
+		require('normalize-package-data')(json);
+	}
+
+	return json;
+};
+
+module.exports.sync = options => {
+	options = {
+		cwd: process.cwd(),
+		normalize: true,
+		...options
+	};
+
+	const filePath = path.resolve(options.cwd, 'package.json');
+	const json = parseJson(fs.readFileSync(filePath, 'utf8'));
+
+	if (options.normalize) {
+		require('normalize-package-data')(json);
+	}
+
+	return json;
+};