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/reusify/benchmarks/reuseNoCodeFunction.js

@@ -0,0 +1,38 @@
+'use strict'
+
+var reusify = require('../')
+var fib = require('./fib')
+var instance = reusify(MyObject)
+var max = 100000000
+var start = Date.now()
+
+function reuseNoCodeFunction () {
+  var obj = instance.get()
+  obj.num = 100
+  obj.func()
+  obj.num = 0
+  instance.release(obj)
+}
+
+function MyObject () {
+  this.next = null
+  var that = this
+  this.num = 0
+  this.func = function () {
+    /* eslint no-constant-condition: "off" */
+    if (null) {
+      // do nothing
+    } else {
+      fib(that.num)
+    }
+  }
+}
+
+for (var i = 0; i < max; i++) {
+  reuseNoCodeFunction()
+}
+
+var time = Date.now() - start
+console.log('Total time', time)
+console.log('Total iterations', max)
+console.log('Iteration/s', max / time * 1000)