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/mkdirp-classic/README.md

@@ -0,0 +1,18 @@
+# mkdirp-classic
+
+Just a non-deprecated mirror of [mkdirp 0.5.2](https://github.com/substack/node-mkdirp/tree/0.5.1)
+for use in modules where we depend on the non promise interface.
+
+```
+npm install mkdirp-classic
+```
+
+## Usage
+
+``` js
+// See the above link
+```
+
+## License
+
+MIT