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/once/once.js

@@ -0,0 +1,42 @@
+var wrappy = require('wrappy')
+module.exports = wrappy(once)
+module.exports.strict = wrappy(onceStrict)
+
+once.proto = once(function () {
+  Object.defineProperty(Function.prototype, 'once', {
+    value: function () {
+      return once(this)
+    },
+    configurable: true
+  })
+
+  Object.defineProperty(Function.prototype, 'onceStrict', {
+    value: function () {
+      return onceStrict(this)
+    },
+    configurable: true
+  })
+})
+
+function once (fn) {
+  var f = function () {
+    if (f.called) return f.value
+    f.called = true
+    return f.value = fn.apply(this, arguments)
+  }
+  f.called = false
+  return f
+}
+
+function onceStrict (fn) {
+  var f = function () {
+    if (f.called)
+      throw new Error(f.onceError)
+    f.called = true
+    return f.value = fn.apply(this, arguments)
+  }
+  var name = fn.name || 'Function wrapped with `once`'
+  f.onceError = name + " shouldn't be called more than once"
+  f.called = false
+  return f
+}