feat(desktop): show activity timestamps on demand (#1506)

Signed-off-by: Taylor Ho <taylorkmho@gmail.com>
Co-authored-by: npub1223z34hd7vtwc6qj4s7flsxkj644nlre2nthu7lrrmkumhu3xddsrx9r6w <52a228d6edf316ec6812ac3c9fc0d696ab59fc7954d77e7be31eedcddf91335b@sprout-oss.stage.blox.sqprod.co>

desktop/src/features/agents/ui/transcriptTimestampPreference.ts

@@ -0,0 +1,62 @@
+import * as React from "react";
+
+/**
+ * User preference for showing a timestamp under each activity row — polished
+ * transcript rows and raw JSON-RPC cards alike. Persisted in localStorage and
+ * shared across every transcript surface, same as the animation preference.
+ * This is a device-level UI preference, not workspace-scoped data, so it is
+ * intentionally not reset on workspace switch. Defaults off to keep the feed
+ * compact.
+ */
+const STORAGE_KEY = "buzz:show-transcript-timestamps";
+
+const listeners = new Set<() => void>();
+
+let timestampsEnabled = readStoredPreference();
+
+function readStoredPreference(): boolean {
+  if (typeof window === "undefined") {
+    return false;
+  }
+
+  try {
+    return window.localStorage.getItem(STORAGE_KEY) === "1";
+  } catch {
+    return false;
+  }
+}
+
+function subscribe(listener: () => void): () => void {
+  listeners.add(listener);
+  return () => {
+    listeners.delete(listener);
+  };
+}
+
+function getSnapshot(): boolean {
+  return timestampsEnabled;
+}
+
+function getServerSnapshot(): boolean {
+  return false;
+}
+
+/** Update the preference and notify all subscribed components. */
+export function setTranscriptTimestampsEnabled(enabled: boolean): void {
+  timestampsEnabled = enabled;
+
+  try {
+    window.localStorage.setItem(STORAGE_KEY, enabled ? "1" : "0");
+  } catch {
+    // Persistence is best-effort; the in-memory value still applies.
+  }
+
+  for (const listener of listeners) {
+    listener();
+  }
+}
+
+/** Whether polished activity rows should show a timestamp footer. */
+export function useTranscriptTimestampsEnabled(): boolean {
+  return React.useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot);
+}