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/css-tree/lib/syntax/node/Selector.js

@@ -0,0 +1,32 @@
+export const name = 'Selector';
+export const structure = {
+    children: [[
+        'TypeSelector',
+        'IdSelector',
+        'ClassSelector',
+        'AttributeSelector',
+        'PseudoClassSelector',
+        'PseudoElementSelector',
+        'Combinator',
+        'WhiteSpace'
+    ]]
+};
+
+export function parse() {
+    const children = this.readSequence(this.scope.Selector);
+
+    // nothing were consumed
+    if (this.getFirstListNode(children) === null) {
+        this.error('Selector is expected');
+    }
+
+    return {
+        type: 'Selector',
+        loc: this.getLocationFromList(children),
+        children
+    };
+}
+
+export function generate(node) {
+    this.children(node);
+}