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/Url.js

@@ -0,0 +1,52 @@
+import * as url from '../../utils/url.js';
+import * as string from '../../utils/string.js';
+import {
+    Function as FunctionToken,
+    String as StringToken,
+    Url,
+    RightParenthesis
+} from '../../tokenizer/index.js';
+
+export const name = 'Url';
+export const structure = {
+    value: String
+};
+
+// <url-token> | <function-token> <string> )
+export function parse() {
+    const start = this.tokenStart;
+    let value;
+
+    switch (this.tokenType) {
+        case Url:
+            value = url.decode(this.consume(Url));
+            break;
+
+        case FunctionToken:
+            if (!this.cmpStr(this.tokenStart, this.tokenEnd, 'url(')) {
+                this.error('Function name must be `url`');
+            }
+
+            this.eat(FunctionToken);
+            this.skipSC();
+            value = string.decode(this.consume(StringToken));
+            this.skipSC();
+            if (!this.eof) {
+                this.eat(RightParenthesis);
+            }
+            break;
+
+        default:
+            this.error('Url or Function is expected');
+    }
+
+    return {
+        type: 'Url',
+        loc: this.getLocation(start, this.tokenStart),
+        value
+    };
+}
+
+export function generate(node) {
+    this.token(Url, url.encode(node.value));
+}