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/istanbul-lib-report/lib/report-base.js

@@ -0,0 +1,16 @@
+'use strict';
+
+// TODO: switch to class private field when targetting node.js 12
+const _summarizer = Symbol('ReportBase.#summarizer');
+
+class ReportBase {
+    constructor(opts = {}) {
+        this[_summarizer] = opts.summarizer;
+    }
+
+    execute(context) {
+        context.getTree(this[_summarizer]).visit(this, context);
+    }
+}
+
+module.exports = ReportBase;