Show latency percentile bands instead of a single-quantile picker

The quantile selector on duration/transfer/db-* panels shows one of p50/p95/p99 at a time. A shaded p50–p99 band with a p95 line shows the distribution shape in one view and makes tail blowups visible without toggling. The backing records already carry all percentile columns (that's what the picker switches between); rendering is a Recharts `` between two series. The picker could remain as a "focus" control or be dropped.

## Acceptance criteria
- In aggregate view, duration/transfer/db-read/db-write/db-message panels render a shaded p50–p99 band with a solid p95 line; tooltip lists p50/p95/p99 at the hovered time.
- Per-dimension (per-table/per-path) view keeps the current single-quantile picker behavior — bands only where there's one series to band.
- Colors follow the existing series palette with reduced-opacity fill; the band is distinguishable in both themes.

## Implementation notes
- The backing records already carry all percentile columns (`pipeline/quantileFields.ts` — it's what the picker switches between).
- Recharts: render an `Area` for the band (p50 as `baseValue`/stacked trick or a range Area) beneath the p95 `Line`; consider a `bandFields` extension on `MetricSpec` so it stays spec-driven.

Filed from a Status tab code review.

Comment generated by kAIle (Claude Fable 5)