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/string-width/index.d.ts

@@ -0,0 +1,29 @@
+declare const stringWidth: {
+	/**
+	Get the visual width of a string - the number of columns required to display it.
+
+	Some Unicode characters are [fullwidth](https://en.wikipedia.org/wiki/Halfwidth_and_fullwidth_forms) and use double the normal width. [ANSI escape codes](https://en.wikipedia.org/wiki/ANSI_escape_code) are stripped and doesn't affect the width.
+
+	@example
+	```
+	import stringWidth = require('string-width');
+
+	stringWidth('a');
+	//=> 1
+
+	stringWidth('古');
+	//=> 2
+
+	stringWidth('\u001B[1m古\u001B[22m');
+	//=> 2
+	```
+	*/
+	(string: string): number;
+
+	// TODO: remove this in the next major version, refactor the whole definition to:
+	// declare function stringWidth(string: string): number;
+	// export = stringWidth;
+	default: typeof stringWidth;
+}
+
+export = stringWidth;