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/default-gateway/android.js

@@ -0,0 +1,43 @@
+"use strict";
+
+const {isIP} = require("net");
+const execa = require("execa");
+
+const args = {
+  v4: ["-4", "r"],
+  v6: ["-6", "r"],
+};
+
+const parse = stdout => {
+  let result;
+
+  (stdout || "").trim().split("\n").some(line => {
+    const [_, gateway, iface] = /default via (.+?) dev (.+?)( |$)/.exec(line) || [];
+    if (gateway && isIP(gateway)) {
+      result = {gateway, interface: (iface ? iface : null)};
+      return true;
+    }
+  });
+
+  if (!result) {
+    throw new Error("Unable to determine default gateway");
+  }
+
+  return result;
+};
+
+const promise = async family => {
+  const {stdout} = await execa("ip", args[family]);
+  return parse(stdout, family);
+};
+
+const sync = family => {
+  const {stdout} = execa.sync("ip", args[family]);
+  return parse(stdout);
+};
+
+module.exports.v4 = () => promise("v4");
+module.exports.v6 = () => promise("v6");
+
+module.exports.v4.sync = () => sync("v4");
+module.exports.v6.sync = () => sync("v6");