Fix session file and elapsed replay state

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

@@ -1320,7 +1320,7 @@ const ActivityChainSummary: FC<{
 			? Math.max(0, Math.round(value))
 			: null;
 	});
-	const [elapsedMs, setElapsedMs] = useState(storedElapsedMs ?? 0);
+	const [elapsedMs, setElapsedMs] = useState<number | null>(storedElapsedMs);
 	const label = useAuiState((s) => {
 		const message = s.message;
 		const status =
@@ -1338,7 +1338,7 @@ const ActivityChainSummary: FC<{
 
 	useEffect(() => {
 		if (!active) {
-			if (storedElapsedMs !== null) setElapsedMs(storedElapsedMs);
+			setElapsedMs(storedElapsedMs);
 			return;
 		}
 		const startedAt =
@@ -1392,7 +1392,7 @@ function findNumberInActivityParts(
 function summarizeActivityChainLabel(
 	content: readonly ActivitySummaryPart[],
 	status: string | undefined,
-	elapsedMs = 0,
+	elapsedMs: number | null = null,
 ) {
 	const toolParts = content.filter((part) => part.type === "tool-call");
 	const runningTool = toolParts.findLast((part) => part.result === undefined);
@@ -1412,7 +1412,7 @@ function summarizeActivityChainLabel(
 		(isGenericActivityLine(lastReasoningLine) ? "" : lastReasoningLine);
 
 	if (status === "running") {
-		const elapsed = `用时 ${formatActivityDuration(elapsedMs)}`;
+		const elapsed = `用时 ${formatActivityDuration(elapsedMs ?? 0)}`;
 		if (activeDetail) {
 			return `${compactActivityLabel(activeDetail, 15)}，${elapsed}`;
 		}
@@ -1424,8 +1424,10 @@ function summarizeActivityChainLabel(
 		15,
 	);
 	const finalDuration = extractDurationLabel(lastReasoningLine);
-	const elapsed = finalDuration || formatActivityDuration(elapsedMs);
-	return `${label}，用时 ${elapsed}`;
+	const elapsed =
+		finalDuration ||
+		(elapsedMs === null ? "" : formatActivityDuration(elapsedMs));
+	return elapsed ? `${label}，用时 ${elapsed}` : label;
 }
 
 function getToolStageNote(value: unknown) {