feat(compare): readable comparison chart on mobile

The compare chart squashed clustered schools into a thin band (y pinned
0-100) under an in-chart title + per-school legend that ate ~40% of a
300px card, leaving converging lines indistinguishable on phones.

- Auto-fit the y-axis to the data on all viewports (computeYBounds in
  lib/utils: padded + min-span for percentages, symmetric around 0 for
  progress, fitted for scores; negative pct-named trend metrics are not
  zero-clamped).
- Distinct point style per school (circle/triangle/rect/rectRot/star)
  as secondary encoding for convergence and colour-blindness.
- Mobile: drop in-chart title/legend/axis titles; add a chip row (colour
  dot + name) that doubles as tap-to-focus — highlights one school's
  line and dims the rest. Chart card 300px -> 340px, nearly all plot.
- Fix a latent colour mismatch: datasets were built from Object.entries
  whose integer-like URN keys enumerate in ascending numeric order,
  desyncing line colours from card colours; the chart now receives the
  ordered school list.
- Union years across schools instead of taking the first school's.
- Extract PerformanceChart's matchMedia pattern into hooks/useIsMobile.

Unit tests for metricKind/computeYBounds; e2e journey covers the mobile
chips and focus toggle.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

nextjs-app/components/PerformanceChart.tsx

@@ -10,12 +10,13 @@
 
 'use client';
 
-import { useEffect, useMemo, useState } from 'react';
+import { useMemo, useState } from 'react';
 import { Line } from 'react-chartjs-2';
 import { ChartOptions, ChartDataset } from 'chart.js';
 import '@/lib/chartSetup';
 import type { SchoolResult } from '@/lib/types';
 import { formatAcademicYear } from '@/lib/utils';
+import { useIsMobile } from '@/hooks/useIsMobile';
 import { track } from '@/lib/analytics';
 import styles from './PerformanceChart.module.css';
 
@@ -68,16 +69,7 @@ export function PerformanceChart({
   const sortedData = [...data].sort((a, b) => a.year - b.year);
   const years = sortedData.map(d => formatAcademicYear(d.year));
 
-  // ── Mobile detection ─────────────────────────────────────────────────
-  // Hydration-safe: SSR renders desktop; client flips to mobile after mount.
-  const [isMobile, setIsMobile] = useState(false);
-  useEffect(() => {
-    const mq = window.matchMedia('(max-width: 640px)');
-    const update = () => setIsMobile(mq.matches);
-    update();
-    mq.addEventListener('change', update);
-    return () => mq.removeEventListener('change', update);
-  }, []);
+  const isMobile = useIsMobile();
 
   // ── Build per-year national averages ─────────────────────────────────
   const natRefRwm: (number | null)[] = sortedData.map(d => {