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/postcss/lib/postcss.d.mts

@@ -0,0 +1,66 @@
+export {
+  // Type-only exports
+  AcceptedPlugin,
+  AnyNode,
+  atRule,
+  AtRule,
+  AtRuleProps,
+  Builder,
+  ChildNode,
+  ChildProps,
+  comment,
+  Comment,
+  CommentProps,
+  Container,
+  ContainerProps,
+  CssSyntaxError,
+  decl,
+  Declaration,
+  DeclarationProps,
+  // postcss function / namespace
+  default,
+  document,
+  Document,
+  DocumentProps,
+  FilePosition,
+  fromJSON,
+  Helpers,
+  Input,
+  JSONHydrator,
+  // This is a class, but it’s not re-exported. That’s why it’s exported as type-only here.
+  type LazyResult,
+  list,
+  Message,
+  Node,
+  NodeErrorOptions,
+  NodeProps,
+  OldPlugin,
+  parse,
+  Parser,
+  // @ts-expect-error This value exists, but it’s untyped.
+  plugin,
+  Plugin,
+  PluginCreator,
+  Position,
+  Postcss,
+  ProcessOptions,
+  Processor,
+  Result,
+  root,
+  Root,
+  RootProps,
+  rule,
+  Rule,
+  RuleProps,
+  Source,
+  SourceMap,
+  SourceMapOptions,
+  Stringifier,
+  // Value exports from postcss.mjs
+  stringify,
+  Syntax,
+  TransformCallback,
+  Transformer,
+  Warning,
+  WarningOptions
+} from './postcss.js'