feat(session-ui): show when a turn finished in the response footer - #62345

Issue for this PR

Closes #62344

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The response footer shows agent, model and duration on hover, but not when the turn finished. This adds the finish time to the end of that line: a bare time for today (14:32), Yesterday, 23:10, then Nov 4, 9:15, with the year once it differs.

The time is the completion time of the same step the duration measures to, so prompt time + duration = the time shown. session-timeline-row.tsx passes it down as turnCompletedAt; outside the timeline the footer falls back to its own message's completion time, as the duration does.

The user message menu already had the Today / Yesterday / date formatting, so I moved it into message/message-time.ts and use it in both places, with a bareToday option for the footer. Both labels read the day from a shared clock (useToday() in relative-time.ts), so they say Yesterday after midnight in a session left open overnight; before this, the menu kept saying Today. The clock ticks every 30 seconds and when the page is shown or focused again, since timers stop while the machine sleeps. A tick only redraws labels when the date changes, so its cost doesn't grow with the number of messages. Formatting comes from Intl, so there are no new strings to translate. The time doesn't shrink; the model name truncates first in a narrow footer.

I didn't add a feature flag, since this only adds a read-only label to an existing footer and there is no client-side flag path for session-ui today. Happy to gate it if you'd rather.

How did you verify your code works?

• bun run test in packages/session-ui, including a new message-time.test.ts that passes the current day in (just after midnight, yesterday, earlier this year, last year, and bareToday).
• bun typecheck in packages/session-ui and packages/app, and bun run typecheck:tsc in packages/session-ui.
• Component tests session-message-projection.spec.ts and session-timeline-mobile.spec.ts pass. I added a datetime check to the response-speed test; it fails if the footer shows the wrong message's time. A new story test moves Playwright's clock past midnight and expects both labels to say Yesterday; it fails if either label reads the clock only once. Another pauses the clock, moves the date on and fires visibilitychange and focus; it fails if either listener is missing.
• Live: the dev app against an isolated serve, with an imported session whose turns finished two days ago, last night and today; also at 390px wide and with the footer squeezed to check truncation.

Visual

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  <text x="38" y="51" fill="currentColor" fill-opacity=".55">Build · Opus 5.5 · 6m 28s<tspan class="t"> · 14:32</tspan></text>
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Screenshots / recordings

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Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

Added by the opencode bot (AI-generated).

Related: #53192, #35348