feat(mui): add MonthField kit primitive (year+month picker, yyyy-MM string I/O)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/ui/index.ts

@@ -16,6 +16,7 @@ export { Tabs, TabPanel } from "./Tabs";
 export type { TabDef } from "./Tabs";
 export { default as Pagination } from "./Pagination";
 export { default as DateField } from "./DateField";
+export { default as MonthField } from "./MonthField";
 export { default as PageHeader } from "./PageHeader";
 export { default as EmptyState } from "./EmptyState";
 export { default as LoadingState } from "./LoadingState";