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/clipboard/test/common/command.js

@@ -0,0 +1,49 @@
+import select from 'select';
+import command from '../../src/common/command';
+
+describe('#command', () => {
+  before(() => {
+    global.stub = sinon.stub(document, 'execCommand');
+    global.input = document.createElement('input');
+    global.input.setAttribute('id', 'input');
+    global.input.setAttribute('value', 'abc');
+    document.body.appendChild(global.input);
+  });
+
+  after(() => {
+    global.stub.restore();
+    document.body.innerHTML = '';
+  });
+
+  it('should execute cut', (done) => {
+    global.stub.returns(true);
+    select(document.querySelector('#input'));
+
+    assert.isTrue(command('cut'));
+    done();
+  });
+
+  it('should execute copy', (done) => {
+    global.stub.returns(true);
+    select(document.querySelector('#input'));
+
+    assert.isTrue(command('copy'));
+    done();
+  });
+
+  it('should not execute copy', (done) => {
+    global.stub.returns(false);
+    select(document.querySelector('#input'));
+
+    assert.isFalse(command('copy'));
+    done();
+  });
+
+  it('should not execute cut', (done) => {
+    global.stub.returns(false);
+    select(document.querySelector('#input'));
+
+    assert.isFalse(command('cut'));
+    done();
+  });
+});