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/core-js-pure/internals/inspect-source.js

@@ -0,0 +1,15 @@
+'use strict';
+var uncurryThis = require('../internals/function-uncurry-this');
+var isCallable = require('../internals/is-callable');
+var store = require('../internals/shared-store');
+
+var functionToString = uncurryThis(Function.toString);
+
+// this helper broken in `core-js@3.4.1-3.4.4`, so we can't use `shared` helper
+if (!isCallable(store.inspectSource)) {
+  store.inspectSource = function (it) {
+    return functionToString(it);
+  };
+}
+
+module.exports = store.inspectSource;