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/date-fns/docs/constants.md

@@ -0,0 +1,47 @@
+# Constants
+
+date-fns provides with a number of useful constants.
+
+## Usage
+
+The constants could be imported from `date-fns/constants` or directly
+from `date-fns`:
+
+```js
+import { maxTime } from 'date-fns/constants'
+import { minTime } from 'date-fns'
+
+function isAllowedTime(time) {
+  return time <= maxTime && time >= minTime
+}
+```
+
+## Constants
+
+### `maxTime`
+
+Maximum allowed time:
+
+```js
+import { maxTime } from 'date-fns'
+
+const isValid = 8640000000000001 <= maxTime
+//=> false
+
+new Date(8640000000000001)
+//=> Invalid Date
+```
+
+### `minTime`
+
+Minimum allowed time:
+
+```js
+import { minTime } from 'date-fns'
+
+const isValid = -8640000000000001 >= minTime
+//=> false
+
+new Date(-8640000000000001)
+//=> Invalid Date
+```