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/is-buffer/test/basic.js

@@ -0,0 +1,24 @@
+var isBuffer = require('../')
+var test = require('tape')
+
+test('is-buffer', function (t) {
+  t.equal(isBuffer(Buffer.alloc(4)), true, 'new Buffer(4)')
+  t.equal(isBuffer(Buffer.allocUnsafeSlow(100)), true, 'SlowBuffer(100)')
+
+  t.equal(isBuffer(undefined), false, 'undefined')
+  t.equal(isBuffer(null), false, 'null')
+  t.equal(isBuffer(''), false, 'empty string')
+  t.equal(isBuffer(true), false, 'true')
+  t.equal(isBuffer(false), false, 'false')
+  t.equal(isBuffer(0), false, '0')
+  t.equal(isBuffer(1), false, '1')
+  t.equal(isBuffer(1.0), false, '1.0')
+  t.equal(isBuffer('string'), false, 'string')
+  t.equal(isBuffer({}), false, '{}')
+  t.equal(isBuffer([]), false, '[]')
+  t.equal(isBuffer(function foo () {}), false, 'function foo () {}')
+  t.equal(isBuffer({ isBuffer: null }), false, '{ isBuffer: null }')
+  t.equal(isBuffer({ isBuffer: function () { throw new Error() } }), false, '{ isBuffer: function () { throw new Error() } }')
+
+  t.end()
+})