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/eslintrc/universal.js

@@ -0,0 +1,9 @@
+// Jest (and probably some other runtimes with custom implementations of
+// `require`) doesn't support `exports` in `package.json`, so this file is here
+// to help them load this module. Note that it is also `.js` and not `.cjs` for
+// the same reason - `cjs` files requires to be loaded with an extension, but
+// since Jest doesn't respect `module` outside of ESM mode it still works in
+// this case (and the `require` in _this_ file does specify the extension).
+
+// eslint-disable-next-line no-undef
+module.exports = require("./dist/eslintrc-universal.cjs");