[ENHANCEMENT] Add time and date format settings and message timestamps #2221

Problem (one or two sentences)

Zoo Code always writes times and dates the way the browser decides, and chat messages show no time. Users cannot choose a 12-hour or 24-hour clock, and they cannot see when a message was sent.

Context (who is affected and when)

• Users who work in a 24-hour or day-first locale see a format they did not choose in task history.
• Users who read a long chat later cannot tell when a reply arrived.
• The "Auto" choice must follow the display language that the user picked in Settings, not the system locale.

Checked against main: the Language tab has only the display language. ChatRow shows no message time. Task history shows only a relative time.

Desired behavior (conceptual, not technical)

• Language tab: a Time format choice (Auto, 12-hour, 24-hour) and a Date format choice (Auto, MM/DD/YYYY, DD/MM/YYYY, YYYY-MM-DD).
• A Show relative times option for task history. When off, the history shows the absolute date and time.
• A live preview in the Language tab that follows the choices before the user presses Save. "Auto" previews the language that is selected in the picker.
• Each chat message shows when it was sent, in the chosen formats. Assistant replies show the time after they finish streaming.

Constraints / preferences

• Defaults keep today's look: Auto, Auto, relative times on.
• No new network call and no new telemetry.
• New strings are translated in all supported locales.

Acceptance criteria

Given the user picks 24-hour and presses Save,
When they look at a chat message,
Then the time under the message uses the 24-hour clock.

Given the user picks German in the language picker and has not pressed Save,
When Time format is Auto,
Then the preview shows a 24-hour time.

Given a timestamp that is not a valid date,
Then no time is shown and the chat row still renders.