feat(metrics): inline cache hit-rate badges (last turn + chat total)

Derive cache hit rate (cacheReadTokens / inputTokens) from data already
folded in core/metrics — no backend/contract change.

- core/metrics: computeCachePct + viewCacheRate (pct + success/warning/error
  level by 66/33 thresholds + isHit); thread a running cumulativeUsage onto
  each finalized turn-metrics row for the conversation total.
- ChatView: render two labelled, colour-coded percentage badges in the
  turn-total bubble — "Last turn:" (that turn) and "Chat Total:" (cumulative).
- Honour backend caveats: absent cache fields -> 0, divide-by-zero guarded,
  a legitimate 0% renders plainly (not "no data").

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