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/has-bigints/index.js

@@ -0,0 +1,11 @@
+'use strict';
+
+var $BigInt = typeof BigInt !== 'undefined' && BigInt;
+
+/** @type {import('.')} */
+module.exports = function hasNativeBigInts() {
+	return typeof $BigInt === 'function'
+		&& typeof BigInt === 'function'
+		&& typeof $BigInt(42) === 'bigint' // eslint-disable-line no-magic-numbers
+		&& typeof BigInt(42) === 'bigint'; // eslint-disable-line no-magic-numbers
+};