feat(chat): add MessageTimestamp component gated on the preference

surfsense_web/components/assistant-ui/message-timestamp.tsx

@@ -0,0 +1,24 @@
+import { useAuiState } from "@assistant-ui/react";
+import { useAtomValue } from "jotai";
+import type { FC } from "react";
+import { showMessageTimestampsAtom } from "@/atoms/chat/show-timestamps.atom";
+import { formatMessageTimestamp } from "@/lib/format-date";
+import { cn } from "@/lib/utils";
+
+/**
+ * Muted, always-visible timestamp under a chat message. Renders only when the
+ * user has opted in via {@link showMessageTimestampsAtom} and the message
+ * carries a ``createdAt`` (absent on optimistic pre-persist messages).
+ */
+export const MessageTimestamp: FC<{ className?: string }> = ({ className }) => {
+	const show = useAtomValue(showMessageTimestampsAtom);
+	const createdAt = useAuiState(({ message }) => message?.createdAt);
+
+	if (!show || !createdAt) return null;
+
+	return (
+		<div className={cn("select-none text-[11px] text-muted-foreground", className)}>
+			{formatMessageTimestamp(createdAt)}
+		</div>
+	);
+};