feat(scoreboard): contextual top-10 chart — labels + detail tooltip

[tbcsec]

Fixes #52

What

The top-10 bars carried only a rank number — no context without scanning the table below. Each column now explains itself three ways:

• At a glance: points figure above every bar; rank stacked over a truncated name below it (stacked rather than inline per owner feedback mid-build — a tenth-width column is too cramped for both on one line).
• On hover / focus: a detail tooltip — medal RankBadge, full name, points, last solve, division when brackets are in play, and the "you"/"your team" marker.
• To a screen reader: an aria-label per column carrying the same facts, so parity doesn't depend on the visual layer.

Design notes

• React-state reveal, not a CSS :hover/:focus trick. Keyboard Tab and mobile tap (tap = focus on a button) open the tooltip through the same onFocus/onMouseEnter path, one at a time, and the tests assert the reveal itself rather than trusting a pseudo-class. (I built the CSS-only version first and found it unverifiable — neither RTL nor the preview browser can prove a :focus reveal — so I switched to state on purpose.)
• Edge anchoring: the first/last two columns pin their tooltip to the near edge instead of centring, so nothing clips outside the card — measured: first tooltip left 305px vs card left 281px.
• Headroom: bars scale to 86% so the tallest bar's points label always fits.
• Extractions per §14: chart → components/scoreboard/top-chart.tsx, RankBadge → rank-badge.tsx (now shared by the table and the tooltip). Hand-rolled, §9 tokens only, no tooltip library.

Tests & verification

• +7 RTL tests: aria-label facts, mode-worded "you/your team", focus reveal + blur hide, hover one-at-a-time, tooltip content, at-a-glance labels, bar scaling (86% leader / 0% zero-points). Suite 130 passed; tsc + eslint clean; npm run build exit 0.
• Browser (demo board, 10 live entries): tooltip verified open via the React path with correct content; layout screenshotted on a dark and light palette (medal disc + popover legible on both); no console errors; live WS updates re-derive the chart unchanged.

Frontend-only — no backend, no migration.

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