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/@wordpress/eslint-plugin/rules/no-global-active-element.js

@@ -0,0 +1,19 @@
+module.exports = {
+	meta: {
+		type: 'problem',
+		schema: [],
+	},
+	create( context ) {
+		return {
+			'MemberExpression[object.name="document"][property.name="activeElement"]'(
+				node
+			) {
+				context.report( {
+					node,
+					message:
+						'Avoid accessing the active element with a global. Use the ownerDocument property on a node ref instead.',
+				} );
+			},
+		};
+	},
+};