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/w3c-xmlserializer/package.json

@@ -0,0 +1,33 @@
+{
+  "name": "w3c-xmlserializer",
+  "description": "A per-spec XML serializer implementation",
+  "keywords": [
+    "dom",
+    "w3c",
+    "xml",
+    "xmlserializer"
+  ],
+  "version": "4.0.0",
+  "license": "MIT",
+  "dependencies": {
+    "xml-name-validator": "^4.0.0"
+  },
+  "devDependencies": {
+    "@domenic/eslint-config": "^3.0.0",
+    "eslint": "^8.28.0",
+    "jest": "^29.3.1",
+    "jsdom": "^20.0.2"
+  },
+  "repository": "jsdom/w3c-xmlserializer",
+  "files": [
+    "lib/"
+  ],
+  "main": "lib/serialize.js",
+  "scripts": {
+    "test": "jest",
+    "lint": "eslint ."
+  },
+  "engines": {
+    "node": ">=14"
+  }
+}