feat(desktop): show per-turn wall-clock duration in the transcript

[Tommy00748]

Summary

Each assistant reply in the desktop app now carries a small time badge (⏱ 38s) below the message text, showing how long its turn took (message.start → message.complete). Users get a glanceable sense of task latency per turn — useful when a turn spins through many tools or a long reasoning block.

Design notes

• Renderer-side timing. The desktop already tracks turnStartedAt per session (set on message.start, cleared on completion). The badge duration is computed from that at settle time — no backend change, no new events, works for adopted and local turns alike.
• Stamped on ChatMessage.durationS at completion in completeAssistantMessage (all settle paths via completeMessage/newAssistantFromCompletion) and on failAssistantMessage (failed turns get a badge too).
• Not persisted. History hydrated from the backend has no badge, matching how reasoning-block durations already behave (useMeasuredDuration is renderer-memory only). The badge is a "you were here" signal, not archival data.
• Perf-safe. The useAuiState selector for the badge reads a value set once at completion — referentially stable across the 30 Hz delta stream, so no per-token re-renders (the component has strict perf rules; the footer/action bar must not subscribe to streaming text).
• i18n: new assistant.thread.turnDuration key added to all five locale files.

Verification

• npm run typecheck (renderer + electron + e2e tsconfigs) ✅
• eslint clean on all changed files ✅
• npm run test:ui — 413 files / 3680 tests ✅
• vitest run --project electron — 86 files / 1023 tests ✅
• npm run test:desktop:all (packaging + bundle validation) ✅

[alt-glitch]

This was generated by AI during triage.

Related: #81949 is an earlier open implementation of Desktop turn-duration display that persists end-to-end metadata. This PR instead keeps renderer-only wall-clock duration for live turns. The user-facing scope overlaps, but the persistence/design choices differ; maintainer selection is needed.

[teknium1]

Salvaged and merged in #87245 — your commit landed on main with authorship preserved (a6f7a5b → rebase-merged). Two small adjustments during the salvage: kept main's newer timeline fields (completedAt/completeOpenTimelineParts) alongside durationS in the settle paths, and dropped the MessageAge footer component since main had grown MessageTimelineTimestamp in the meantime. The badge itself, the settle-time stamping, the re-render contract entry, and all five locale strings shipped as you wrote them. Nice perf-conscious design — thanks!