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/autoprefixer/lib/hacks/file-selector-button.js

@@ -0,0 +1,26 @@
+let Selector = require('../selector')
+let utils = require('../utils')
+
+class FileSelectorButton extends Selector {
+  constructor(name, prefixes, all) {
+    super(name, prefixes, all)
+
+    if (this.prefixes) {
+      this.prefixes = utils.uniq(this.prefixes.map(() => '-webkit-'))
+    }
+  }
+
+  /**
+   * Return different selectors depend on prefix
+   */
+  prefixed(prefix) {
+    if (prefix === '-webkit-') {
+      return '::-webkit-file-upload-button'
+    }
+    return `::${prefix}file-selector-button`
+  }
+}
+
+FileSelectorButton.names = ['::file-selector-button']
+
+module.exports = FileSelectorButton