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/_default-custom-properties.scss

@@ -0,0 +1,11 @@
+// It is important to include these styles in all built stylesheets.
+// This allows to CSS variables post CSS plugin to generate fallbacks.
+// It also provides default CSS variables for npm package consumers.
+:root {
+	@include admin-scheme(#007cba);
+	--wp-block-synced-color: #7a00df;
+	--wp-block-synced-color--rgb: #{hex-to-rgb(#7a00df)};
+	// This CSS variable is not used in Gutenberg project,
+	// but is maintained for backwards compatibility.
+	--wp-bound-block-color: var(--wp-block-synced-color);
+}