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/async-function/test/index.js

@@ -0,0 +1,40 @@
+'use strict';
+
+var test = require('tape');
+var getProto = require('get-proto');
+var semver = require('semver');
+
+var getAsyncFunction = require('../');
+
+test('getAsyncFunction', function (t) {
+	var result = getAsyncFunction();
+
+	/* eslint-env browser */
+	if (typeof window === 'undefined' && typeof process !== 'undefined') {
+		t.equal(
+			!!result,
+			semver.satisfies(process.version, '>= 7.6'),
+			'result is present or absent as expected for node ' + process.version
+		);
+	}
+
+	t.test('exists', { skip: !result }, function (st) {
+		if (result && getProto) { // TS can't infer `skip`, or that getProto definitely exists if AsyncFunction exists
+			st.equal(typeof result, 'function', 'is a function');
+			st.equal(getProto(result), Function, 'extends Function');
+
+			return result('a', 'return a')(42).then(function (a) {
+				st.equal(a, 42, 'returns an async function');
+			});
+		}
+		return st.fail('should never get here');
+	});
+
+	t.test('does not exist', { skip: !!result }, function (st) {
+		st.equal(result, false, 'is false');
+
+		st.end();
+	});
+
+	t.end();
+});