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/core-js/modules/es.date.to-string.js

@@ -0,0 +1,20 @@
+'use strict';
+// TODO: Remove from `core-js@4`
+var uncurryThis = require('../internals/function-uncurry-this');
+var defineBuiltIn = require('../internals/define-built-in');
+
+var DatePrototype = Date.prototype;
+var INVALID_DATE = 'Invalid Date';
+var TO_STRING = 'toString';
+var nativeDateToString = uncurryThis(DatePrototype[TO_STRING]);
+var thisTimeValue = uncurryThis(DatePrototype.getTime);
+
+// `Date.prototype.toString` method
+// https://tc39.es/ecma262/#sec-date.prototype.tostring
+if (String(new Date(NaN)) !== INVALID_DATE) {
+  defineBuiltIn(DatePrototype, TO_STRING, function toString() {
+    var value = thisTimeValue(this);
+    // eslint-disable-next-line no-self-compare -- NaN check
+    return value === value ? nativeDateToString(this) : INVALID_DATE;
+  });
+}