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-jest/docs/rules/prefer-to-have-length.md

@@ -0,0 +1,39 @@
+# Suggest using `toHaveLength()` (`prefer-to-have-length`)
+
+💼 This rule is enabled in the 🎨 `style`
+[config](https://github.com/jest-community/eslint-plugin-jest/blob/main/README.md#shareable-configurations).
+
+🔧 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 -->
+
+In order to have a better failure message, `toHaveLength()` should be used upon
+asserting expectations on objects length property.
+
+## Rule details
+
+This rule triggers a warning if `toBe()`, `toEqual()` or `toStrictEqual()` is
+used to assert objects length property.
+
+```js
+expect(files.length).toBe(1);
+```
+
+This rule is enabled by default.
+
+The following patterns are considered warnings:
+
+```js
+expect(files.length).toBe(1);
+
+expect(files.length).toEqual(1);
+
+expect(files.length).toStrictEqual(1);
+```
+
+The following pattern is not warning:
+
+```js
+expect(files).toHaveLength(1);
+```