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/events/tests/events-list.js

@@ -0,0 +1,28 @@
+'use strict';
+
+var EventEmitter = require('../');
+var assert = require('assert');
+
+var EE = new EventEmitter();
+var m = function() {};
+EE.on('foo', function() {});
+assert.equal(1, EE.eventNames().length);
+assert.equal('foo', EE.eventNames()[0]);
+EE.on('bar', m);
+assert.equal(2, EE.eventNames().length);
+assert.equal('foo', EE.eventNames()[0]);
+assert.equal('bar', EE.eventNames()[1]);
+EE.removeListener('bar', m);
+assert.equal(1, EE.eventNames().length);
+assert.equal('foo', EE.eventNames()[0]);
+
+if (typeof Symbol !== 'undefined') {
+  var s = Symbol('s');
+  EE.on(s, m);
+  assert.equal(2, EE.eventNames().length);
+  assert.equal('foo', EE.eventNames()[0]);
+  assert.equal(s, EE.eventNames()[1]);
+  EE.removeListener(s, m);
+  assert.equal(1, EE.eventNames().length);
+  assert.equal('foo', EE.eventNames()[0]);
+}