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/eslint-plugin-import/docs/rules/no-nodejs-modules.md

@@ -0,0 +1,42 @@
+# import/no-nodejs-modules
+
+<!-- end auto-generated rule header -->
+
+Forbid the use of Node.js builtin modules. Can be useful for client-side web projects that do not have access to those modules.
+
+## Options
+
+This rule supports the following options:
+
+ - `allow`: Array of names of allowed modules. Defaults to an empty array.
+
+## Rule Details
+
+### Fail
+
+```js
+import fs from 'fs';
+import path from 'path';
+
+var fs = require('fs');
+var path = require('path');
+```
+
+### Pass
+
+```js
+import _ from 'lodash';
+import foo from 'foo';
+import foo from './foo';
+
+var _ = require('lodash');
+var foo = require('foo');
+var foo = require('./foo');
+
+/* eslint import/no-nodejs-modules: ["error", {"allow": ["path"]}] */
+import path from 'path';
+```
+
+## When Not To Use It
+
+If you have a project that is run mainly or partially using Node.js.