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/@wordpress/base-styles/_functions.scss

@@ -0,0 +1,9 @@
+/**
+*  Converts a hex value into the rgb equivalent.
+*
+* @param {string} hex - the hexadecimal value to convert
+* @return {string} comma separated rgb values
+*/
+@function hex-to-rgb($hex) {
+	@return red($hex), green($hex), blue($hex);
+}