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/through/test/buffering.js

@@ -0,0 +1,71 @@
+var test = require('tape')
+var through = require('../')
+
+// must emit end before close.
+
+test('buffering', function(assert) {
+  var ts = through(function (data) {
+    this.queue(data)
+  }, function () {
+    this.queue(null)
+  })
+
+  var ended = false,  actual = []
+
+  ts.on('data', actual.push.bind(actual))
+  ts.on('end', function () {
+    ended = true
+  })
+
+  ts.write(1)
+  ts.write(2)
+  ts.write(3)
+  assert.deepEqual(actual, [1, 2, 3])
+  ts.pause()
+  ts.write(4)
+  ts.write(5)
+  ts.write(6)
+  assert.deepEqual(actual, [1, 2, 3])
+  ts.resume()
+  assert.deepEqual(actual, [1, 2, 3, 4, 5, 6])
+  ts.pause()
+  ts.end()
+  assert.ok(!ended)
+  ts.resume()
+  assert.ok(ended)
+  assert.end()
+})
+
+test('buffering has data in queue, when ends', function (assert) {
+
+  /*
+   * If stream ends while paused with data in the queue,
+   * stream should still emit end after all data is written
+   * on resume.
+   */
+
+  var ts = through(function (data) {
+    this.queue(data)
+  }, function () {
+    this.queue(null)
+  })
+
+  var ended = false,  actual = []
+
+  ts.on('data', actual.push.bind(actual))
+  ts.on('end', function () {
+    ended = true
+  })
+
+  ts.pause()
+  ts.write(1)
+  ts.write(2)
+  ts.write(3)
+  ts.end()
+  assert.deepEqual(actual, [], 'no data written yet, still paused')
+  assert.ok(!ended, 'end not emitted yet, still paused')
+  ts.resume()
+  assert.deepEqual(actual, [1, 2, 3], 'resumed, all data should be delivered')
+  assert.ok(ended, 'end should be emitted once all data was delivered')
+  assert.end();
+})