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/fraction.js/examples/angles.js

@@ -0,0 +1,26 @@
+/*
+Fraction.js v5.0.0 10/1/2024
+https://raw.org/article/rational-numbers-in-javascript/
+
+Copyright (c) 2024, Robert Eisele (https://raw.org/)
+Licensed under the MIT license.
+*/
+
+// This example generates a list of angles with human readable radians
+
+var Fraction = require('fraction.js');
+
+var tab = [];
+for (var d = 1; d <= 360; d++) {
+
+   var pi = Fraction(2, 360).mul(d);
+   var tau = Fraction(1, 360).mul(d);
+
+   if (pi.d <= 6n && pi.d != 5n)
+      tab.push([
+         d,
+         pi.toFraction() + "pi",
+         tau.toFraction() + "tau"]);
+}
+
+console.table(tab);