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-absolute-path.md

@@ -0,0 +1,54 @@
+# import/no-absolute-path
+
+🔧 This rule is automatically fixable by the [`--fix` CLI option](https://eslint.org/docs/latest/user-guide/command-line-interface#--fix).
+
+<!-- end auto-generated rule header -->
+
+Node.js allows the import of modules using an absolute path such as `/home/xyz/file.js`. That is a bad practice as it ties the code using it to your computer, and therefore makes it unusable in packages distributed on `npm` for instance.
+
+This rule forbids the import of modules using absolute paths.
+
+## Rule Details
+
+### Fail
+
+```js
+import f from '/foo';
+import f from '/some/path';
+
+var f = require('/foo');
+var f = require('/some/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');
+```
+
+### Options
+
+By default, only ES6 imports and CommonJS `require` calls will have this rule enforced.
+
+You may provide an options object providing true/false for any of
+
+ - `esmodule`: defaults to `true`
+ - `commonjs`: defaults to `true`
+ - `amd`: defaults to `false`
+
+If `{ amd: true }` is provided, dependency paths for AMD-style `define` and `require`
+calls will be resolved:
+
+```js
+/*eslint import/no-absolute-path: [2, { commonjs: false, amd: true }]*/
+define(['/foo'], function (foo) { /*...*/ }) // reported
+require(['/foo'], function (foo) { /*...*/ }) // reported
+
+const foo = require('/foo') // ignored because of explicit `commonjs: false`
+```