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/object-inspect/test/has.js

@@ -0,0 +1,15 @@
+'use strict';
+
+var inspect = require('../');
+var test = require('tape');
+var mockProperty = require('mock-property');
+
+test('when Object#hasOwnProperty is deleted', function (t) {
+    t.plan(1);
+    var arr = [1, , 3]; // eslint-disable-line no-sparse-arrays
+
+    t.teardown(mockProperty(Array.prototype, 1, { value: 2 })); // this is needed to account for "in" vs "hasOwnProperty"
+    t.teardown(mockProperty(Object.prototype, 'hasOwnProperty', { 'delete': true }));
+
+    t.equal(inspect(arr), '[ 1, , 3 ]');
+});