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/eslint/messages/invalid-rule-severity.js

@@ -0,0 +1,13 @@
+"use strict";
+
+const { stringifyValueForError } = require("./shared");
+
+module.exports = function({ ruleId, value }) {
+    return `
+Configuration for rule "${ruleId}" is invalid. Expected severity of "off", 0, "warn", 1, "error", or 2.
+
+You passed '${stringifyValueForError(value, 4)}'.
+
+See https://eslint.org/docs/latest/use/configure/rules#using-configuration-files for configuring rules.
+`.trimStart();
+};