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/unique-string/index.d.ts

@@ -0,0 +1,16 @@
+/**
+Generate a unique random string.
+
+@returns A 32 character unique string. Matches the length of MD5, which is [unique enough](https://stackoverflow.com/a/2444336/64949) for non-crypto purposes.
+
+@example
+```
+import uniqueString = require('unique-string');
+
+uniqueString();
+//=> 'b4de2a49c8ffa3fbee04446f045483b2'
+```
+*/
+declare function uniqueString(): string;
+
+export = uniqueString;