show indicator of stream expiring soon

js/src/hooks/use-expiring-soon.ts

@@ -0,0 +1,20 @@
+import { useMemo } from "react";
+
+/**
+ * Returns true when a stream is within the last 10% of its retention window.
+ * For example, with 24h retention, this fires when < 2.4h remain.
+ */
+export function useExpiringSoon(
+  lastChildCreatedAt: Date | undefined,
+  retentionHours: number,
+): boolean {
+  return useMemo(() => {
+    if (!lastChildCreatedAt) return false;
+
+    const retentionMs = retentionHours * 60 * 60 * 1000;
+    const expiresAt = lastChildCreatedAt.getTime() + retentionMs;
+    const remaining = expiresAt - Date.now();
+
+    return remaining > 0 && remaining < retentionMs * 0.1;
+  }, [lastChildCreatedAt, retentionHours]);
+}