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/requireindex/test/lib/.also_private/private.txt

@@ -0,0 +1 @@
+barf

node_modules/requireindex/test/lib/Foo.js

@@ -0,0 +1,4 @@
+module.exports = {
+  l: function() { return "yes"; },
+  ls: function() { return "yep"; }
+};

node_modules/requireindex/test/lib/_private.js

@@ -0,0 +1,3 @@
+module.exports = function() {
+  return "ack";
+}

node_modules/requireindex/test/lib/bam.js

@@ -0,0 +1,4 @@
+module.exports = {
+  m: function() { return "ok"; },
+  n: require('./_private')
+};

node_modules/requireindex/test/lib/bar/f.js

@@ -0,0 +1,3 @@
+module.exports = function() {
+  return "yea";
+}

node_modules/requireindex/test/lib/bar/fed/again.js

@@ -0,0 +1,3 @@
+module.exports = function() {
+  return "again";
+}

node_modules/requireindex/test/lib/bar/fed/ignored.js

@@ -0,0 +1,3 @@
+module.exports = function() {
+  return "ignored";
+}

node_modules/requireindex/test/lib/bar/fed/index.js

@@ -0,0 +1 @@
+module.exports = require('../../../../')(__dirname, ['again', 'somemore'])

node_modules/requireindex/test/lib/bar/fed/somemore.js

@@ -0,0 +1,3 @@
+module.exports = function() {
+  return "somemore";
+}

node_modules/requireindex/test/lib/bar/fing.js

@@ -0,0 +1,3 @@
+module.exports = function() {
+  return "definitely";
+}

node_modules/requireindex/test/lib/bar/index.js

@@ -0,0 +1 @@
+module.exports = require('../../../')(__dirname)

node_modules/requireindex/test/lib/index.js

@@ -0,0 +1 @@
+module.exports = require('../../')(__dirname)

node_modules/requireindex/test/lib/not_javascript.txt

@@ -0,0 +1 @@
+asdf 1 2 / @ 123