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/icons/src/icon/index.js

@@ -0,0 +1,27 @@
+/**
+ * WordPress dependencies
+ */
+import { cloneElement, forwardRef } from '@wordpress/element';
+
+/** @typedef {{icon: JSX.Element, size?: number} & import('@wordpress/primitives').SVGProps} IconProps */
+
+/**
+ * Return an SVG icon.
+ *
+ * @param {IconProps}                                 props icon is the SVG component to render
+ *                                                          size is a number specifiying the icon size in pixels
+ *                                                          Other props will be passed to wrapped SVG component
+ * @param {import('react').ForwardedRef<HTMLElement>} ref   The forwarded ref to the SVG element.
+ *
+ * @return {JSX.Element}  Icon component
+ */
+function Icon( { icon, size = 24, ...props }, ref ) {
+	return cloneElement( icon, {
+		width: size,
+		height: size,
+		...props,
+		ref,
+	} );
+}
+
+export default forwardRef( Icon );

node_modules/@wordpress/icons/src/icon/stories/index.story.js

@@ -0,0 +1,94 @@
+/**
+ * WordPress dependencies
+ */
+import { useState, Fragment } from '@wordpress/element';
+
+/**
+ * Internal dependencies
+ */
+import Icon from '../';
+import check from '../../library/check';
+import * as icons from '../../';
+
+const { Icon: _Icon, ...availableIcons } = icons;
+
+const meta = {
+	component: Icon,
+	title: 'Icons/Icon',
+	parameters: {
+		controls: { hideNoControlsWarning: true },
+	},
+};
+export default meta;
+
+export const Default = () => {
+	return (
+		<>
+			<div>
+				<h2>Dashicons (corrected viewport)</h2>
+
+				<Icon icon={ check } />
+				<Icon icon={ check } size={ 36 } />
+				<Icon icon={ check } size={ 48 } />
+			</div>
+			<div>
+				<h2>Material and Other</h2>
+
+				<Icon icon={ icons.paragraph } />
+				<Icon icon={ icons.paragraph } size={ 36 } />
+				<Icon icon={ icons.paragraph } size={ 48 } />
+			</div>
+		</>
+	);
+};
+
+const LibraryExample = () => {
+	const [ filter, setFilter ] = useState( '' );
+	const filteredIcons = filter.length
+		? Object.fromEntries(
+				Object.entries( availableIcons ).filter( ( [ name ] ) =>
+					name.includes( filter )
+				)
+		  )
+		: availableIcons;
+	return (
+		<div style={ { padding: 20 } }>
+			<label htmlFor="filter-icon" style={ { paddingRight: 10 } }>
+				Filter Icons
+			</label>
+			<input
+				// eslint-disable-next-line no-restricted-syntax
+				id="filter-icons"
+				type="search"
+				value={ filter }
+				placeholder="Icon name"
+				onChange={ ( event ) => setFilter( event.target.value ) }
+			/>
+			<div style={ { marginTop: 20 } }>
+				<div
+					style={ {
+						display: 'inline-grid',
+						alignItems: 'center',
+						gap: 4,
+						gridTemplateColumns: 'auto 24px 36px 48px',
+					} }
+				>
+					{ Object.entries( filteredIcons ).map(
+						( [ name, icon ] ) => {
+							return (
+								<Fragment key={ name }>
+									<strong>{ name }</strong>
+									<Icon icon={ icon } />
+									<Icon icon={ icon } size={ 36 } />
+									<Icon icon={ icon } size={ 48 } />
+								</Fragment>
+							);
+						}
+					) }
+				</div>
+			</div>
+		</div>
+	);
+};
+
+export const Library = () => <LibraryExample />;