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/rational-pow.js

@@ -0,0 +1,29 @@
+/*
+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.
+*/
+const Fraction = require('fraction.js');
+ 
+// Calculates (a/b)^(c/d) if result is rational
+// Derivation: https://raw.org/book/analysis/rational-numbers/
+function root(a, b, c, d) {
+
+  // Initial estimate
+  let x = Fraction(100 * (Math.floor(Math.pow(a / b, c / d)) || 1), 100);
+  const abc = Fraction(a, b).pow(c);
+
+  for (let i = 0; i < 30; i++) {
+    const n = abc.mul(x.pow(1 - d)).sub(x).div(d).add(x)
+
+    if (x.n === n.n && x.d === n.d) {
+      return n;
+    }
+    x = n;
+  }
+  return null;
+}
+
+root(18, 2, 1, 2); // 3/1