feat: Add TPS and TTFT display to step finish view

[kkito]

Summary

Add TPS (throughput / tokens per second) and TTFT (time to first token) display to the step finish view, along with settings toggles to control their visibility.

Changes

New display metrics (StepFinishPartView.tsx)

• Show TPS (tokens/sec) computed from API timestamps and generation tokens
• Show TTFT (time to first token) when firstPartStart is available
• Accept new props: firstPartStart, tps, ttft

Settings toggles (ChatSettings.tsx, themeStore.ts)

• Added showTps toggle — enable/disable TPS display
• Added showTtft toggle — enable/disable TTFT display

i18n (locales/en/settings.json, locales/zh-CN/settings.json)

• Added tps / showTps entries
• Added ttft / showTtft entries

Infrastructure

• MessageRenderer.tsx: extract firstPartStart from conversation timestamps and pass it down
• Display unit uses tps (lowercase) instead of T/s

Files changed

 .../plans/2026-06-08-tps-ttft-display.md           | 580 ++++++++
 .../specs/2026-06-08-tps-ttft-display-design.md    |  94 +++
 src/features/message/MessageRenderer.tsx           |  20 +
 .../message/parts/StepFinishPartView.test.tsx      | 136 +++++
 src/features/message/parts/StepFinishPartView.tsx  |  20 +
 src/features/settings/components/ChatSettings.tsx  |   2 +
 src/locales/en/settings.json                       |   4 +
 src/locales/zh-CN/settings.json                    |   4 +
 src/store/themeStore.ts                            |   4 +
 9 files changed, 864 insertions(+)

Notes

• TPS is computed based on opencode API timestamps, includes LLM network overhead