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/object.assign/hasSymbols.js

@@ -0,0 +1,43 @@
+'use strict';
+
+var keys = require('object-keys');
+
+module.exports = function hasSymbols() {
+	if (typeof Symbol !== 'function' || typeof Object.getOwnPropertySymbols !== 'function') { return false; }
+	if (typeof Symbol.iterator === 'symbol') { return true; }
+
+	var obj = {};
+	var sym = Symbol('test');
+	var symObj = Object(sym);
+	if (typeof sym === 'string') { return false; }
+
+	if (Object.prototype.toString.call(sym) !== '[object Symbol]') { return false; }
+	if (Object.prototype.toString.call(symObj) !== '[object Symbol]') { return false; }
+
+	/*
+	 * temp disabled per https://github.com/ljharb/object.assign/issues/17
+	 * if (sym instanceof Symbol) { return false; }
+	 * temp disabled per https://github.com/WebReflection/get-own-property-symbols/issues/4
+	 * if (!(symObj instanceof Symbol)) { return false; }
+	 */
+
+	var symVal = 42;
+	obj[sym] = symVal;
+	for (sym in obj) { return false; } // eslint-disable-line no-unreachable-loop
+	if (keys(obj).length !== 0) { return false; }
+	if (typeof Object.keys === 'function' && Object.keys(obj).length !== 0) { return false; }
+
+	if (typeof Object.getOwnPropertyNames === 'function' && Object.getOwnPropertyNames(obj).length !== 0) { return false; }
+
+	var syms = Object.getOwnPropertySymbols(obj);
+	if (syms.length !== 1 || syms[0] !== sym) { return false; }
+
+	if (!Object.prototype.propertyIsEnumerable.call(obj, sym)) { return false; }
+
+	if (typeof Object.getOwnPropertyDescriptor === 'function') {
+		var descriptor = Object.getOwnPropertyDescriptor(obj, sym);
+		if (descriptor.value !== symVal || descriptor.enumerable !== true) { return false; }
+	}
+
+	return true;
+};