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/resolve/test/resolver/multirepo/lerna.json

@@ -0,0 +1,6 @@
+{
+  "packages": [
+    "packages/*"
+  ],
+  "version": "0.0.0"
+}

node_modules/resolve/test/resolver/multirepo/package.json

@@ -0,0 +1,20 @@
+{
+  "name": "ljharb-monorepo-symlink-test",
+  "private": true,
+  "version": "0.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "postinstall": "lerna bootstrap",
+    "test": "node packages/package-a"
+  },
+  "author": "",
+  "license": "MIT",
+  "dependencies": {
+    "jquery": "^3.3.1",
+    "resolve": "../../../"
+  },
+  "devDependencies": {
+    "lerna": "^3.4.3"
+  }
+}

node_modules/resolve/test/resolver/multirepo/packages/package-a/index.js

@@ -0,0 +1,35 @@
+'use strict';
+
+var assert = require('assert');
+var path = require('path');
+var resolve = require('resolve');
+
+var basedir = __dirname + '/node_modules/@my-scope/package-b';
+
+var expected = path.join(__dirname, '../../node_modules/jquery/dist/jquery.js');
+
+/*
+ * preserveSymlinks === false
+ * will search NPM package from
+ * - packages/package-b/node_modules
+ * - packages/node_modules
+ * - node_modules
+ */
+assert.equal(resolve.sync('jquery', { basedir: basedir, preserveSymlinks: false }), expected);
+assert.equal(resolve.sync('../../node_modules/jquery', { basedir: basedir, preserveSymlinks: false }), expected);
+
+/*
+ * preserveSymlinks === true
+ * will search NPM package from
+ * - packages/package-a/node_modules/@my-scope/packages/package-b/node_modules
+ * - packages/package-a/node_modules/@my-scope/packages/node_modules
+ * - packages/package-a/node_modules/@my-scope/node_modules
+ * - packages/package-a/node_modules/node_modules
+ * - packages/package-a/node_modules
+ * - packages/node_modules
+ * - node_modules
+ */
+assert.equal(resolve.sync('jquery', { basedir: basedir, preserveSymlinks: true }), expected);
+assert.equal(resolve.sync('../../../../../node_modules/jquery', { basedir: basedir, preserveSymlinks: true }), expected);
+
+console.log(' * all monorepo paths successfully resolved through symlinks');

node_modules/resolve/test/resolver/multirepo/packages/package-a/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "@my-scope/package-a",
+  "version": "0.0.0",
+  "private": true,
+  "description": "",
+  "license": "MIT",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: run tests from root\" && exit 1"
+  },
+  "dependencies": {
+    "@my-scope/package-b": "^0.0.0"
+  }
+}

node_modules/resolve/test/resolver/multirepo/packages/package-b/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "@my-scope/package-b",
+  "private": true,
+  "version": "0.0.0",
+  "description": "",
+  "license": "MIT",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: run tests from root\" && exit 1"
+  },
+  "dependencies": {
+    "@my-scope/package-a": "^0.0.0"
+  }
+}