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/eslint-plugin-import/docs/rules/no-self-import.md

@@ -0,0 +1,32 @@
+# import/no-self-import
+
+<!-- end auto-generated rule header -->
+
+Forbid a module from importing itself. This can sometimes happen during refactoring.
+
+## Rule Details
+
+### Fail
+
+```js
+// foo.js
+import foo from './foo';
+
+const foo = require('./foo');
+```
+
+```js
+// index.js
+import index from '.';
+
+const index = require('.');
+```
+
+### Pass
+
+```js
+// foo.js
+import bar from './bar';
+
+const bar = require('./bar');
+```