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/parse-cache-control/test.js

@@ -0,0 +1,31 @@
+var test = require('tape');
+var parseCacheControl = require('./index');
+
+test('parseCacheControl', function (t) {
+  var header = parseCacheControl('must-revalidate, max-age=3600');
+  t.ok(header);
+  t.equal(header['must-revalidate'], true);
+  t.equal(header['max-age'], 3600);
+
+  header = parseCacheControl('must-revalidate, max-age="3600"');
+  t.ok(header);
+  t.equal(header['must-revalidate'], true);
+  t.equal(header['max-age'], 3600);
+
+  header = parseCacheControl('must-revalidate, b =3600');
+  t.notOk(header);
+
+  header = parseCacheControl('must-revalidate, max-age=a3600');
+  t.notOk(header);
+
+  header = parseCacheControl(123);
+  t.notOk(header);
+
+  header = parseCacheControl(null);
+  t.notOk(header);
+
+  header = parseCacheControl(undefined);
+  t.notOk(header);
+
+  t.end();
+});