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/select/src/select.js

@@ -0,0 +1,43 @@
+function select(element) {
+    var selectedText;
+
+    if (element.nodeName === 'SELECT') {
+        element.focus();
+
+        selectedText = element.value;
+    }
+    else if (element.nodeName === 'INPUT' || element.nodeName === 'TEXTAREA') {
+        var isReadOnly = element.hasAttribute('readonly');
+
+        if (!isReadOnly) {
+            element.setAttribute('readonly', '');
+        }
+
+        element.select();
+        element.setSelectionRange(0, element.value.length);
+
+        if (!isReadOnly) {
+            element.removeAttribute('readonly');
+        }
+
+        selectedText = element.value;
+    }
+    else {
+        if (element.hasAttribute('contenteditable')) {
+            element.focus();
+        }
+
+        var selection = window.getSelection();
+        var range = document.createRange();
+
+        range.selectNodeContents(element);
+        selection.removeAllRanges();
+        selection.addRange(range);
+
+        selectedText = selection.toString();
+    }
+
+    return selectedText;
+}
+
+module.exports = select;