Memoize `formatRelativeTime` per thread item in thread-list

[MODSetter]

Description

In thread-list.tsx, formatRelativeTime(new Date(thread.updatedAt)) runs per thread item per render. This creates a new Date() inside the function for every item on every render. For long thread lists, this adds up.

File to change

• surfsense_web/components/assistant-ui/thread-list.tsx (line 240, function at lines 280+)

Current code

<p className="truncate text-xs text-muted-foreground">
  {formatRelativeTime(new Date(thread.updatedAt))}
</p>

function formatRelativeTime(date: Date): string {
  const now = new Date(); // Created per call
  const diffMs = now.getTime() - date.getTime();
  // ...
}

What to do

Option A: Memoize per item using the already-memoized ThreadListItemComponent:

const relativeTime = useMemo(
  () => formatRelativeTime(new Date(thread.updatedAt)),
  [thread.updatedAt]
);

Option B: Share a single now timestamp across all items via a custom hook with a ticking timer (updates every minute):

function useNow(intervalMs = 60_000) {
  const [now, setNow] = useState(() => new Date());
  useEffect(() => {
    const id = setInterval(() => setNow(new Date()), intervalMs);
    return () => clearInterval(id);
  }, [intervalMs]);
  return now;
}

Option A is simpler and sufficient for a good first issue.

Acceptance criteria

• formatRelativeTime is memoized per thread item
• Relative times still display correctly
• Thread list scrolling feels responsive