feat(mui): kit — Select, StatusChip, CheckboxField, Alert

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

src/admin/ui/Alert.tsx

@@ -0,0 +1,19 @@
+import type { ReactNode } from "react";
+import MuiAlert from "@mui/material/Alert";
+
+/** Inline contextual alert (form/section feedback) — distinct from the global toast. */
+export default function Alert({
+  severity = "info",
+  children,
+  onClose,
+}: {
+  severity?: "success" | "error" | "warning" | "info";
+  children: ReactNode;
+  onClose?: () => void;
+}) {
+  return (
+    <MuiAlert severity={severity} onClose={onClose} sx={{ borderRadius: 2 }}>
+      {children}
+    </MuiAlert>
+  );
+}