Adds a new timestamp next to each message, with a tooltip revealing the
fully expanded date string. The new code uses
`Intl.DateTimeFormat().resolvedOptions().timeZone` to determine the
user's timezone without requiring user input. Timestamps are displayed
in the user’s current time zone, regardless of the time zone in which
the conversation took place.

Fixes an issue where timestamps for optimistic messages had a different
timestamp format (milliseconds) than the timestamps provided by OpenAI
(seconds).

<img width="773" alt="image"
src="https://github.com/user-attachments/assets/c6e807e7-f9f0-4f97-aa93-82cc3900d751"
/>