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/css-tree/lib/syntax/atrule/index.js

@@ -0,0 +1,15 @@
+import fontFace from './font-face.js';
+import importAtrule from './import.js';
+import media from './media.js';
+import nest from './nest.js';
+import page from './page.js';
+import supports from './supports.js';
+
+export default {
+    'font-face': fontFace,
+    'import': importAtrule,
+    media,
+    nest,
+    page,
+    supports
+};