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/MonthField.tsx

@@ -0,0 +1,40 @@
+import { DatePicker } from "@mui/x-date-pickers/DatePicker";
+import { parseISO, format, isValid } from "date-fns";
+
+/**
+ * Month input over MUI X DatePicker (year + month views only). String in /
+ * string out ("yyyy-MM", or ""), matching the legacy AdminDatePicker mode="month"
+ * so it drops into existing form/filter state. cs locale is provided by the
+ * LocalizationProvider in MuiProvider.
+ */
+export default function MonthField({
+  value,
+  onChange,
+  label,
+  disabled,
+  minDate,
+  maxDate,
+}: {
+  value: string;
+  onChange: (value: string) => void;
+  label?: string;
+  disabled?: boolean;
+  minDate?: Date;
+  maxDate?: Date;
+}) {
+  const parsed = value ? parseISO(`${value}-01`) : null;
+  return (
+    <DatePicker
+      label={label}
+      disabled={disabled}
+      minDate={minDate}
+      maxDate={maxDate}
+      views={["year", "month"]}
+      openTo="month"
+      value={parsed && isValid(parsed) ? parsed : null}
+      onChange={(d) => onChange(d && isValid(d) ? format(d, "yyyy-MM") : "")}
+      format="MM.yyyy"
+      slotProps={{ textField: { size: "small", fullWidth: true } }}
+    />
+  );
+}

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";