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/symbols.js

@@ -0,0 +1,25 @@
+'use strict';
+
+var common = require('./common');
+var EventEmitter = require('../');
+var assert = require('assert');
+
+var ee = new EventEmitter();
+var foo = Symbol('foo');
+var listener = common.mustCall();
+
+ee.on(foo, listener);
+assert.strictEqual(ee.listeners(foo).length, 1);
+assert.strictEqual(ee.listeners(foo)[0], listener);
+
+ee.emit(foo);
+
+ee.removeAllListeners();
+assert.strictEqual(ee.listeners(foo).length, 0);
+
+ee.on(foo, listener);
+assert.strictEqual(ee.listeners(foo).length, 1);
+assert.strictEqual(ee.listeners(foo)[0], listener);
+
+ee.removeListener(foo, listener);
+assert.strictEqual(ee.listeners(foo).length, 0);