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/json2php/lib/json2php.js

@@ -0,0 +1,66 @@
+// Generated by CoffeeScript 2.7.0
+var json2php, make,
+  hasProp = {}.hasOwnProperty;
+
+make = function({linebreak = '', indent = '', shortArraySyntax = false} = {}) {
+  var arrClose, arrOpen, nest, transform;
+  arrOpen = shortArraySyntax ? '[' : 'array(';
+  arrClose = shortArraySyntax ? ']' : ')';
+  nest = {
+    '[object Array]': function(obj, parentIndent) {
+      var i, len, results, value;
+      results = [];
+      for (i = 0, len = obj.length; i < len; i++) {
+        value = obj[i];
+        results.push(transform(value, parentIndent));
+      }
+      return results;
+    },
+    '[object Object]': function(obj, parentIndent) {
+      var key, results, value;
+      results = [];
+      for (key in obj) {
+        if (!hasProp.call(obj, key)) continue;
+        value = obj[key];
+        results.push(transform(key, parentIndent) + ' => ' + transform(value, parentIndent));
+      }
+      return results;
+    }
+  };
+  return transform = function(obj, parentIndent = '') {
+    var items, nestIndent, objType, result;
+    objType = Object.prototype.toString.call(obj);
+    switch (objType) {
+      case '[object Null]':
+      case '[object Undefined]':
+        result = 'null';
+        break;
+      case '[object String]':
+        result = "'" + obj.replace(/\\/g, '\\\\').replace(/\'/g, "\\'") + "'";
+        break;
+      case '[object Number]':
+      case '[object Boolean]':
+        result = obj.toString();
+        break;
+      case '[object Array]':
+      case '[object Object]':
+        nestIndent = parentIndent + indent;
+        items = nest[objType](obj, nestIndent);
+        result = `${arrOpen}${linebreak + nestIndent}${items.join(',' + (linebreak === '' ? ' ' : linebreak + nestIndent))}${linebreak + parentIndent}${arrClose}`;
+        break;
+      default:
+        result = 'null';
+    }
+    return result;
+  };
+};
+
+json2php = make();
+
+json2php.make = make;
+
+if (typeof module !== 'undefined' && module.exports) {
+  module.exports = json2php;
+  // Not that good but useful
+  global.json2php = json2php;
+}