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/lodash/_flatRest.js

@@ -0,0 +1,16 @@
+var flatten = require('./flatten'),
+    overRest = require('./_overRest'),
+    setToString = require('./_setToString');
+
+/**
+ * A specialized version of `baseRest` which flattens the rest array.
+ *
+ * @private
+ * @param {Function} func The function to apply a rest parameter to.
+ * @returns {Function} Returns the new function.
+ */
+function flatRest(func) {
+  return setToString(overRest(func, undefined, flatten), func + '');
+}
+
+module.exports = flatRest;