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/Nth.js

@@ -0,0 +1,47 @@
+import { Ident } from '../../tokenizer/index.js';
+
+export const name = 'Nth';
+export const structure = {
+    nth: ['AnPlusB', 'Identifier'],
+    selector: ['SelectorList', null]
+};
+
+export function parse() {
+    this.skipSC();
+
+    const start = this.tokenStart;
+    let end = start;
+    let selector = null;
+    let nth;
+
+    if (this.lookupValue(0, 'odd') || this.lookupValue(0, 'even')) {
+        nth = this.Identifier();
+    } else {
+        nth = this.AnPlusB();
+    }
+
+    end = this.tokenStart;
+    this.skipSC();
+
+    if (this.lookupValue(0, 'of')) {
+        this.next();
+
+        selector = this.SelectorList();
+        end = this.tokenStart;
+    }
+
+    return {
+        type: 'Nth',
+        loc: this.getLocation(start, end),
+        nth,
+        selector
+    };
+}
+
+export function generate(node) {
+    this.node(node.nth);
+    if (node.selector !== null) {
+        this.token(Ident, 'of');
+        this.node(node.selector);
+    }
+}