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/hpack.js/lib/hpack/utils.js

@@ -0,0 +1,25 @@
+exports.assert = function assert(cond, text) {
+  if (!cond)
+    throw new Error(text);
+};
+
+exports.stringify = function stringify(arr) {
+  var res = '';
+  for (var i = 0; i < arr.length; i++)
+    res += String.fromCharCode(arr[i]);
+  return res;
+};
+
+exports.toArray = function toArray(str) {
+  var res = [];
+  for (var i = 0; i < str.length; i++) {
+    var c = str.charCodeAt(i);
+    var hi = c >>> 8;
+    var lo = c & 0xff;
+    if (hi)
+      res.push(hi, lo);
+    else
+      res.push(lo);
+  }
+  return res;
+};