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/symbol-tree/lib/SymbolTreeNode.js

@@ -0,0 +1,54 @@
+'use strict';
+
+module.exports = class SymbolTreeNode {
+        constructor() {
+                this.parent = null;
+                this.previousSibling = null;
+                this.nextSibling = null;
+
+                this.firstChild = null;
+                this.lastChild = null;
+
+                /** This value is incremented anytime a children is added or removed */
+                this.childrenVersion = 0;
+                /** The last child object which has a cached index */
+                this.childIndexCachedUpTo = null;
+
+                /** This value represents the cached node index, as long as
+                 * cachedIndexVersion matches with the childrenVersion of the parent */
+                this.cachedIndex = -1;
+                this.cachedIndexVersion = NaN; // NaN is never equal to anything
+        }
+
+        get isAttached() {
+                return Boolean(this.parent || this.previousSibling || this.nextSibling);
+        }
+
+        get hasChildren() {
+                return Boolean(this.firstChild);
+        }
+
+        childrenChanged() {
+                /* jshint -W016 */
+                // integer wrap around
+                this.childrenVersion = (this.childrenVersion + 1) & 0xFFFFFFFF;
+                this.childIndexCachedUpTo = null;
+        }
+
+        getCachedIndex(parentNode) {
+                // (assumes parentNode is actually the parent)
+                if (this.cachedIndexVersion !== parentNode.childrenVersion) {
+                        this.cachedIndexVersion = NaN;
+                        // cachedIndex is no longer valid
+                        return -1;
+                }
+
+                return this.cachedIndex; // -1 if not cached
+        }
+
+        setCachedIndex(parentNode, index) {
+                // (assumes parentNode is actually the parent)
+                this.cachedIndexVersion = parentNode.childrenVersion;
+                this.cachedIndex = index;
+        }
+};