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/select-hose/test/fixtures.js

@@ -0,0 +1,14 @@
+var assert = require('assert');
+
+function expectData(stream, expected, callback) {
+  var actual = '';
+
+  stream.on('data', function(chunk) {
+    actual += chunk;
+  });
+  stream.on('end', function() {
+    assert.equal(actual, expected);
+    callback();
+  });
+}
+exports.expectData = expectData;