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/utils-merge/package.json

@@ -0,0 +1,40 @@
+{
+  "name": "utils-merge",
+  "version": "1.0.1",
+  "description": "merge() utility function",
+  "keywords": [
+    "util"
+  ],
+  "author": {
+    "name": "Jared Hanson",
+    "email": "jaredhanson@gmail.com",
+    "url": "http://www.jaredhanson.net/"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git://github.com/jaredhanson/utils-merge.git"
+  },
+  "bugs": {
+    "url": "http://github.com/jaredhanson/utils-merge/issues"
+  },
+  "license": "MIT",
+  "licenses": [
+    {
+      "type": "MIT",
+      "url": "http://opensource.org/licenses/MIT"
+    }
+  ],
+  "main": "./index",
+  "dependencies": {},
+  "devDependencies": {
+    "make-node": "0.3.x",
+    "mocha": "1.x.x",
+    "chai": "1.x.x"
+  },
+  "engines": {
+    "node": ">= 0.4.0"
+  },
+  "scripts": {
+    "test": "node_modules/.bin/mocha --reporter spec --require test/bootstrap/node test/*.test.js"
+  }
+}