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/readable-stream/readable.js

@@ -0,0 +1,16 @@
+var Stream = require('stream');
+if (process.env.READABLE_STREAM === 'disable' && Stream) {
+  module.exports = Stream.Readable;
+  Object.assign(module.exports, Stream);
+  module.exports.Stream = Stream;
+} else {
+  exports = module.exports = require('./lib/_stream_readable.js');
+  exports.Stream = Stream || exports;
+  exports.Readable = exports;
+  exports.Writable = require('./lib/_stream_writable.js');
+  exports.Duplex = require('./lib/_stream_duplex.js');
+  exports.Transform = require('./lib/_stream_transform.js');
+  exports.PassThrough = require('./lib/_stream_passthrough.js');
+  exports.finished = require('./lib/internal/streams/end-of-stream.js');
+  exports.pipeline = require('./lib/internal/streams/pipeline.js');
+}