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/web-vitals/dist/modules/onFCP.d.ts

@@ -0,0 +1,10 @@
+import { FCPReportCallback, MetricRatingThresholds, ReportOpts } from './types.js';
+/** Thresholds for FCP. See https://web.dev/articles/fcp#what_is_a_good_fcp_score */
+export declare const FCPThresholds: MetricRatingThresholds;
+/**
+ * Calculates the [FCP](https://web.dev/articles/fcp) value for the current page and
+ * calls the `callback` function once the value is ready, along with the
+ * relevant `paint` performance entry used to determine the value. The reported
+ * value is a `DOMHighResTimeStamp`.
+ */
+export declare const onFCP: (onReport: FCPReportCallback, opts?: ReportOpts) => void;