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/pend/README.md

@@ -0,0 +1,41 @@
+# Pend
+
+Dead-simple optimistic async helper.
+
+## Usage
+
+```js
+var Pend = require('pend');
+var pend = new Pend();
+pend.max = 10; // defaults to Infinity
+setTimeout(pend.hold(), 1000); // pend.wait will have to wait for this hold to finish
+pend.go(function(cb) {
+  console.log("this function is immediately executed");
+  setTimeout(function() {
+    console.log("calling cb 1");
+    cb();
+  }, 500);
+});
+pend.go(function(cb) {
+  console.log("this function is also immediately executed");
+  setTimeout(function() {
+    console.log("calling cb 2");
+    cb();
+  }, 1000);
+});
+pend.wait(function(err) {
+  console.log("this is excuted when the first 2 have returned.");
+  console.log("err is a possible error in the standard callback style.");
+});
+```
+
+Output:
+
+```
+this function is immediately executed
+this function is also immediately executed
+calling cb 1
+calling cb 2
+this is excuted when the first 2 have returned.
+err is a possible error in the standard callback style.
+```