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/is-generator-fn/index.d.ts

@@ -0,0 +1,24 @@
+declare const isGeneratorFn: {
+	/**
+	Check if something is a generator function.
+
+	@example
+	```
+	import isGeneratorFn = require('is-generator-fn');
+
+	isGeneratorFn(function * () {});
+	//=> true
+
+	isGeneratorFn(function () {});
+	//=> false
+	```
+	*/
+	(value: unknown): value is GeneratorFunction;
+
+	// TODO: Remove this for the next major release, refactor the whole definition to:
+	// declare function isGeneratorFn(value: unknown): value is GeneratorFunction;
+	// export = isGeneratorFn;
+	default: typeof isGeneratorFn;
+};
+
+export = isGeneratorFn;