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/markdownlint/doc/Prettier.md

@@ -0,0 +1,23 @@
+# Using markdownlint with Prettier
+
+[`Prettier`](https://prettier.io) is a popular code formatter. For the most part,
+it works seamlessly with `markdownlint`. Special situations are documented
+below.
+
+## List item indentation
+
+The default settings of `markdownlint` and `Prettier` are compatible and don't
+result in any linting violations. If `Prettier` is used with `--tab-width` set
+to `4`, the following `markdownlint` configuration can be used:
+
+```json
+{
+  "MD007": {
+    "indent": 4
+  },
+  "MD030": {
+    "ul_single": 3,
+    "ul_multi": 3
+  }
+}
+```