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/test/index.js

@@ -0,0 +1,44 @@
+'use strict';
+
+var test = require('tape');
+var hasBigInts = require('..');
+
+test('interface', function (t) {
+	t.equal(typeof hasBigInts, 'function', 'is a function');
+	t.equal(typeof hasBigInts(), 'boolean', 'returns a boolean');
+	t.end();
+});
+
+test('BigInts are supported', { skip: !hasBigInts() }, function (t) {
+	t.equal(typeof BigInt, 'function', 'global BigInt is a function');
+	if (typeof BigInt !== 'function') {
+		return;
+	}
+
+	t.equal(BigInt(42), BigInt(42), '42n === 42n');
+	t['throws'](
+		function () { BigInt(NaN); },
+		RangeError,
+		'NaN is not an integer; BigInt(NaN) throws'
+	);
+
+	t['throws'](
+		function () { BigInt(Infinity); },
+		RangeError,
+		'Infinity is not an integer; BigInt(Infinity) throws'
+	);
+
+	t['throws'](
+		function () { BigInt(1.1); },
+		RangeError,
+		'1.1 is not an integer; BigInt(1.1) throws'
+	);
+
+	t.end();
+});
+
+test('BigInts are not supported', { skip: hasBigInts() }, function (t) {
+	t.equal(typeof BigInt, 'undefined', 'global BigInt is undefined');
+
+	t.end();
+});