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/lookup-closest-locale/index.js

@@ -0,0 +1,15 @@
+// @flow
+// "lookup" algorithm http://tools.ietf.org/html/rfc4647#section-3.4
+// assumes normalized language tags, and matches in a case sensitive manner
+module.exports = function lookupClosestLocale (locale/*: string | string[] | void */, available/*: { [string]: any } */)/*: ?string */ {
+  if (typeof locale === 'string' && available[locale]) return locale
+  var locales = [].concat(locale || [])
+  for (var l = 0, ll = locales.length; l < ll; ++l) {
+    var current = locales[l].split('-')
+    while (current.length) {
+      var candidate = current.join('-')
+      if (available[candidate]) return candidate
+      current.pop()
+    }
+  }
+}