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/sum.js

@@ -0,0 +1,24 @@
+var baseSum = require('./_baseSum'),
+    identity = require('./identity');
+
+/**
+ * Computes the sum of the values in `array`.
+ *
+ * @static
+ * @memberOf _
+ * @since 3.4.0
+ * @category Math
+ * @param {Array} array The array to iterate over.
+ * @returns {number} Returns the sum.
+ * @example
+ *
+ * _.sum([4, 2, 8, 6]);
+ * // => 20
+ */
+function sum(array) {
+  return (array && array.length)
+    ? baseSum(array, identity)
+    : 0;
+}
+
+module.exports = sum;