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/hard-rejection/index.d.ts

@@ -0,0 +1,15 @@
+declare const hardRejection: {
+	/**
+	Make unhandled promise rejections fail hard right away instead of the default [silent fail](https://gist.github.com/benjamingr/0237932cee84712951a2).
+
+	@param log - Custom logging function to print the rejected promise. Receives the error stack. Default: `console.error`.
+	*/
+	(log?: (stack?: string) => void): void;
+
+	// TODO: Remove this for the next major release, refactor the whole definition to:
+	// declare function hardRejection(log?: (stack?: string) => void): void;
+	// export = hardRejection;
+	default: typeof hardRejection;
+};
+
+export = hardRejection;