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/cssom/lib/MediaList.js

@@ -0,0 +1,61 @@
+//.CommonJS
+var CSSOM = {};
+///CommonJS
+
+
+/**
+ * @constructor
+ * @see http://dev.w3.org/csswg/cssom/#the-medialist-interface
+ */
+CSSOM.MediaList = function MediaList(){
+	this.length = 0;
+};
+
+CSSOM.MediaList.prototype = {
+
+	constructor: CSSOM.MediaList,
+
+	/**
+	 * @return {string}
+	 */
+	get mediaText() {
+		return Array.prototype.join.call(this, ", ");
+	},
+
+	/**
+	 * @param {string} value
+	 */
+	set mediaText(value) {
+		var values = value.split(",");
+		var length = this.length = values.length;
+		for (var i=0; i<length; i++) {
+			this[i] = values[i].trim();
+		}
+	},
+
+	/**
+	 * @param {string} medium
+	 */
+	appendMedium: function(medium) {
+		if (Array.prototype.indexOf.call(this, medium) === -1) {
+			this[this.length] = medium;
+			this.length++;
+		}
+	},
+
+	/**
+	 * @param {string} medium
+	 */
+	deleteMedium: function(medium) {
+		var index = Array.prototype.indexOf.call(this, medium);
+		if (index !== -1) {
+			Array.prototype.splice.call(this, index, 1);
+		}
+	}
+
+};
+
+
+//.CommonJS
+exports.MediaList = CSSOM.MediaList;
+///CommonJS