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/bl/test/isBufferList.js

@@ -0,0 +1,32 @@
+'use strict'
+
+const tape = require('tape')
+const { BufferList, BufferListStream } = require('../')
+const { Buffer } = require('buffer')
+
+tape('isBufferList positives', (t) => {
+  t.ok(BufferList.isBufferList(new BufferList()))
+  t.ok(BufferList.isBufferList(new BufferListStream()))
+
+  t.end()
+})
+
+tape('isBufferList negatives', (t) => {
+  const types = [
+    null,
+    undefined,
+    NaN,
+    true,
+    false,
+    {},
+    [],
+    Buffer.alloc(0),
+    [Buffer.alloc(0)]
+  ]
+
+  for (const obj of types) {
+    t.notOk(BufferList.isBufferList(obj))
+  }
+
+  t.end()
+})