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-amd.md

@@ -0,0 +1,37 @@
+# import/no-amd
+
+<!-- end auto-generated rule header -->
+
+Reports `require([array], ...)` and `define([array], ...)` function calls at the
+module scope. Will not report if !=2 arguments, or first argument is not a literal array.
+
+Intended for temporary use when migrating to pure ES6 modules.
+
+## Rule Details
+
+This will be reported:
+
+```js
+define(["a", "b"], function (a, b) { /* ... */ })
+
+require(["b", "c"], function (b, c) { /* ... */ })
+```
+
+CommonJS `require` is still valid.
+
+## When Not To Use It
+
+If you don't mind mixing module systems (sometimes this is useful), you probably
+don't want this rule.
+
+It is also fairly noisy if you have a larger codebase that is being transitioned
+from AMD to ES6 modules.
+
+## Contributors
+
+Special thanks to @xjamundx for donating his no-define rule as a start to this.
+
+## Further Reading
+
+ - [`no-commonjs`](./no-commonjs.md): report CommonJS `require` and `exports`
+ - Source: <https://github.com/xjamundx/eslint-plugin-modules>