fix(charts): stop duplicate legend labels + make right legend scroll (MAP-54)

[Makisuo]

Fixes MAP-54 — duplicate labels appearing in chart views (dashboard-polish bug).

What & why

Dashboard widgets host the chart legend in two places:

1. The widget header strip — widget-shell.tsx wraps every chart in a ChartLegendSlotContext.Provider, and ChartContainer auto-publishes config labels into that slot.
2. The in-plot legend.

The default ChartLegendContent returns null when the slot is present (so service charts — latency/throughput/error-rate — are fine), but the custom QueryBuilderLegend used by the query-builder area/line/bar charts never consulted the slot. So with legend: "visible"/"right" and ≥2 series, the series labels rendered twice — once in the widget header, once in the in-plot legend.

Separately, the right-aligned in-plot legend only ever set width, never a height bound, so a long series list overflowed the card instead of scrolling. (The bottom legend was already bounded + scrollable.)

Changes

• ChartContainer: new hoistLegend prop (default true) that gates the legend-slot publish. The three query-builder charts pass hoistLegend={!showLegendBlock} so the header hoist and the in-plot legend become mutually exclusive — header strip when legend is hidden, in-plot legend (with its stats table / per-series toggle) when visible/right. Service charts and non-dashboard usage are unchanged.
• QueryBuilderLegend: new maxHeight prop bound to the container height for the right layout, so its existing overflow-auto body scrolls instead of overflowing the card.
• widget-lab: added a Line — 50 series (right legend, scrolls) stress scenario (line keeps every series, unlike the bucketing bar) to cover right-legend overflow.

Verification

• bun typecheck — 24/24 pass.
• Browser (/widget-lab, which renders through the real ChartWidget/WidgetFrame slot provider with sample data), DOM-measured:

Case	legend	Header strip	In-plot legend	Scrolls (client/scroll H)
Query Builder Line (default)	hidden	shows p50/p95/p99	none	—
Legend — right (3 series)	right	empty (title only)	p50/p95/p99	fits, no scroll
Line — 50 (bottom)	visible	—	50 series	✅ 108 / 1028
Line — 50 (right) new	right	—	50 series	✅ 262 / 1028, no card overflow

The duplicate is gone (right card: headerLegendItems: [], series only in-plot), the header strip still appears when the chart draws no in-plot legend (no over-suppression), and tall right legends now scroll within the card.

Reviewer notes

• The duplicate only manifests inside the widget shell (the slot provider), so /widget-lab reproduces it but the bare chart registry does not.
• Branched off main (not the session's stale chore/cloudflare-email-service branch, whose email work already merged as feat(email): replace Resend with Cloudflare Email Service #150).

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