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/espree/lib/features.js

@@ -0,0 +1,27 @@
+/**
+ * @fileoverview The list of feature flags supported by the parser and their default
+ *      settings.
+ * @author Nicholas C. Zakas
+ */
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+// None!
+
+//------------------------------------------------------------------------------
+// Public
+//------------------------------------------------------------------------------
+
+export default {
+
+    // React JSX parsing
+    jsx: false,
+
+    // allow return statement in global scope
+    globalReturn: false,
+
+    // allow implied strict mode
+    impliedStrict: false
+};