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/postcss-scss/README.md

@@ -0,0 +1,23 @@
+# PostCSS SCSS Syntax
+
+<img align="right" width="95" height="95"
+     title="Philosopher’s stone, logo of PostCSS"
+     src="http://postcss.github.io/postcss/logo.svg">
+
+A [SCSS] parser for [PostCSS].
+
+**This module does not compile SCSS.** It simply parses mixins as custom
+at-rules & variables as properties, so that PostCSS plugins can then transform
+SCSS source code alongside CSS.
+
+[PostCSS]: https://github.com/postcss/postcss
+[SCSS]:    http://sass-lang.com/
+
+<a href="https://evilmartians.com/?utm_source=postcss">
+  <img src="https://evilmartians.com/badges/sponsored-by-evil-martians.svg"
+    alt="Sponsored by Evil Martians" width="236" height="54">
+</a>
+
+
+## Docs
+Read full docs **[here](https://github.com/postcss/postcss-scss#readme)**.