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/is-shared-array-buffer/index.js

@@ -0,0 +1,24 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+/** @type {undefined | ((thisArg: SharedArrayBuffer) => number)} */
+var $byteLength = callBound('SharedArrayBuffer.prototype.byteLength', true);
+
+/** @type {import('.')} */
+module.exports = $byteLength
+	? function isSharedArrayBuffer(obj) {
+		if (!obj || typeof obj !== 'object') {
+			return false;
+		}
+		try {
+			// @ts-expect-error TS can't figure out this closed-over variable is non-nullable, and it's fine that `obj` might not be a SAB
+			$byteLength(obj);
+			return true;
+		} catch (e) {
+			return false;
+		}
+	}
+	: function isSharedArrayBuffer(_obj) { // eslint-disable-line no-unused-vars
+		return false;
+	};