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/grid-start.js

@@ -0,0 +1,33 @@
+let Declaration = require('../declaration')
+
+class GridStart extends Declaration {
+  /**
+   * Do not add prefix for unsupported value in IE
+   */
+  check(decl) {
+    let value = decl.value
+    return !value.includes('/') && !value.includes('span')
+  }
+
+  /**
+   * Return a final spec property
+   */
+  normalize(prop) {
+    return prop.replace('-start', '')
+  }
+
+  /**
+   * Change property name for IE
+   */
+  prefixed(prop, prefix) {
+    let result = super.prefixed(prop, prefix)
+    if (prefix === '-ms-') {
+      result = result.replace('-start', '')
+    }
+    return result
+  }
+}
+
+GridStart.names = ['grid-row-start', 'grid-column-start']
+
+module.exports = GridStart