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-relative-packages.md

@@ -0,0 +1,69 @@
+# import/no-relative-packages
+
+🔧 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 -->
+
+Use this rule to prevent importing packages through relative paths.
+
+It's useful in Yarn/Lerna workspaces, where it's possible to import a sibling package using `../package` relative path, while direct `package` is the correct one.
+
+## Examples
+
+Given the following folder structure:
+
+```pt
+my-project
+├── packages
+│   ├── foo
+│   │   ├── index.js
+│   │   └── package.json
+│   └── bar
+│       ├── index.js
+│       └── package.json
+└── entry.js
+```
+
+And the .eslintrc file:
+
+```json
+{
+  ...
+  "rules": {
+    "import/no-relative-packages": "error"
+  }
+}
+```
+
+The following patterns are considered problems:
+
+```js
+/**
+ *  in my-project/packages/foo.js
+ */
+
+import bar from '../bar'; // Import sibling package using relative path
+import entry from '../../entry.js'; // Import from parent package using relative path
+
+/**
+ *  in my-project/entry.js
+ */
+
+import bar from './packages/bar'; // Import child package using relative path
+```
+
+The following patterns are NOT considered problems:
+
+```js
+/**
+ *  in my-project/packages/foo.js
+ */
+
+import bar from 'bar'; // Import sibling package using package name
+
+/**
+ *  in my-project/entry.js
+ */
+
+import bar from 'bar'; // Import sibling package using package name
+```