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/date-fns/esm/_lib/addLeadingZeros/index.js

@@ -0,0 +1,8 @@
+export default function addLeadingZeros(number, targetLength) {
+  var sign = number < 0 ? '-' : '';
+  var output = Math.abs(number).toString();
+  while (output.length < targetLength) {
+    output = '0' + output;
+  }
+  return sign + output;
+}