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/json-buffer/README.md

@@ -0,0 +1,24 @@
+# json-buffer
+
+JSON functions that can convert buffers!
+
+[![build status](https://secure.travis-ci.org/dominictarr/json-buffer.png)](http://travis-ci.org/dominictarr/json-buffer)
+
+[![testling badge](https://ci.testling.com/dominictarr/json-buffer.png)](https://ci.testling.com/dominictarr/json-buffer)
+
+JSON mangles buffers by converting to an array...
+which isn't helpful. json-buffers converts to base64 instead,
+and deconverts base64 to a buffer.
+
+``` js
+var JSONB = require('json-buffer')
+var Buffer = require('buffer').Buffer
+
+var str = JSONB.stringify(Buffer.from('hello there!'))
+
+console.log(JSONB.parse(str)) //GET a BUFFER back
+```
+
+## License
+
+MIT