Persist active run state for replay

frontend/app/components/assistant-ui/thread.tsx

@@ -1884,7 +1884,6 @@ const ActivityChainSummary: FC<{
 	partIndex: number | undefined;
 }> = ({ messageId, partIndex }) => {
 	const { openItem } = useActivityPanel();
-	const startedAtRef = useRef(Date.now());
 	const metadataRunStartedAtMs = useAuiState((s) => {
 		const value = readMetadataValue(s.message.metadata, "runStartedAtMs");
 		return typeof value === "number" && Number.isFinite(value) ? value : null;
@@ -1933,9 +1932,11 @@ const ActivityChainSummary: FC<{
 		const startedAt =
 			metadataRunStartedAtMs ??
 			contentRunStartedAtMs ??
-			(storedElapsedMs === null
-				? startedAtRef.current
-				: Date.now() - storedElapsedMs);
+			(storedElapsedMs === null ? null : Date.now() - storedElapsedMs);
+		if (startedAt === null) {
+			setElapsedMs(storedElapsedMs);
+			return;
+		}
 		setElapsedMs(Math.max(0, Date.now() - startedAt));
 		const timer = window.setInterval(() => {
 			setElapsedMs(Math.max(0, Date.now() - startedAt));
@@ -2021,11 +2022,12 @@ function summarizeActivityChainLabel(
 		(isGenericActivityLine(lastReasoningLine) ? "" : lastReasoningLine);
 
 	if (status === "running") {
-		const elapsed = `用时 ${formatActivityDuration(elapsedMs ?? 0)}`;
+		const elapsed =
+			elapsedMs === null ? "" : `，用时 ${formatActivityDuration(elapsedMs)}`;
 		if (activeDetail) {
-			return `${compactActivityLabel(activeDetail, 15)}，${elapsed}`;
+			return `${compactActivityLabel(activeDetail, 15)}${elapsed}`;
 		}
-		return `思考中，${elapsed}`;
+		return elapsed ? `思考中${elapsed}` : "思考中";
 	}
 
 	const label = compactActivityLabel(