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/comment-parser/tests/e2e/examples.spec.js

@@ -0,0 +1,16 @@
+const {
+  parse,
+  stringify,
+  transforms,
+  tokenizers,
+} = require('../../lib/index.cjs');
+const { examples } = require('./examples');
+
+const table = examples.map((fn) => [fn.name.replace(/_/g, ' '), fn]);
+
+test.each(table)('example - %s', (name, fn) => {
+  const source = fn.toString();
+  expect(() =>
+    fn(source, parse, stringify, transforms, tokenizers)
+  ).not.toThrow();
+});