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/fast-diff/README.md

@@ -0,0 +1,24 @@
+# Fast Diff ![Build Status](https://github.com/jhchen/fast-diff/actions/workflows/test.yml/badge.svg)
+
+This is a simplified import of the excellent [diff-match-patch](https://code.google.com/p/google-diff-match-patch/) library by [Neil Fraser](https://neil.fraser.name/) into the Node.js environment. The match and patch parts are removed, as well as all the extra diff options. What remains is incredibly fast diffing between two strings.
+
+ The diff function is an implementation of ["An O(ND) Difference Algorithm and its Variations" (Myers, 1986)](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.4.6927&rep=rep1&type=pdf) with the suggested divide and conquer strategy along with several [optimizations](http://neil.fraser.name/news/2007/10/09/) Neil added.
+
+```js
+var diff = require('fast-diff');
+
+var good = 'Good dog';
+var bad = 'Bad dog';
+
+var result = diff(good, bad);
+// [[-1, "Goo"], [1, "Ba"], [0, "d dog"]]
+
+// Respect suggested edit location (cursor position), added in v1.1
+diff('aaa', 'aaaa', 1)
+// [[0, "a"], [1, "a"], [0, "aa"]]
+
+// For convenience
+diff.INSERT === 1;
+diff.EQUAL === 0;
+diff.DELETE === -1;
+```