Show latest message or settlement date in the sidebar thread hover card

[Wiktor102]

The sidebar's relative age alone isn't enough to identify when work last happened or when a thread was settled. An exact date and time in the existing thread hover card would make this easy to check without opening the conversation.

Please show:

• For active threads: Latest message, followed by the date and time of the most recent message, whether from the user or the assistant.
• For settled threads: Settled, followed by the date and time the thread was settled.

Use local date/time formatting and respect the user's time-format preference. Clear labels would distinguish message activity from settlement.

Related: #7590 proposes last-activity and prompt-cache timing on hover. This request is specifically about exact message and settlement dates, independent of cache information.