feat(chart): mobile-only single-metric chip selector for Results Over Time

The Results Over Time chart on the school detail page was overcrowded
on phones — 5-item legend wrapping over the plot area, a hidden right
y-axis still rendered for the (collapsed) progress series, fixed
0–100 percentage scale that flattened all variation, angled x-axis
labels eating vertical space.

At ≤640px the chart now renders one metric at a time, selected via a
chip strip above the plot. No more legend. No more dual y-axis. The
y-axis auto-tightens around the actual data range so variation is
visible (a school sitting in the 70–90% band now uses a 65–95 axis
instead of squashing onto a 0–100 line). A small subtitle above the
chips sets the subject context ("KS2 SATs · Reading, Writing & Maths"
or "GCSE results · Year 11") so chip labels can describe the *view*
rather than re-stating the subject.

Chip labels are spelled out in parent-facing language — no internal
shorthand like "RWM":

  Primary  (KS2):  At expected level (default)
                   Above expected level
                   Pupil progress         (shows 3 series + mini-legend)

  Secondary (KS4): Attainment 8 (default)
                   English & Maths grade 4+
                   Progress 8

Chips disable themselves (greyed, with a "No data for this school"
title) when the underlying series has no data points. Desktop
behaviour is unchanged — the full multi-series chart with dual y-axis
still renders >640px.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nextjs-app/components/PerformanceChart.module.css

@@ -29,8 +29,98 @@
   font-style: italic;
 }
 
+/* ── Mobile chip selector ────────────────────────────────────────────
+   Hidden on desktop. Replaces the in-chart legend on phones — one
+   metric at a time so the line variation is actually readable. */
+
+.mobileChips {
+  display: none;
+}
+
+.mobileSubtitle {
+  font-size: 0.7rem;
+  font-weight: 700;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+  color: var(--text-muted, #6d685f);
+  margin-bottom: 0.4rem;
+}
+
+.chipRow {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.375rem;
+}
+
+.chip {
+  border: 1px solid var(--border-color, #e5dfd5);
+  background: var(--bg-card, #fff);
+  color: var(--text-secondary, #5c564d);
+  padding: 0.4rem 0.75rem;
+  border-radius: 999px;
+  font-size: 0.8125rem;
+  font-weight: 500;
+  font-family: inherit;
+  cursor: pointer;
+  white-space: nowrap;
+  transition: background-color 0.15s ease, border-color 0.15s ease, color 0.15s ease;
+  -webkit-tap-highlight-color: transparent;
+}
+
+.chip:active {
+  background: var(--bg-secondary, #f3ede4);
+}
+
+.chip:disabled {
+  opacity: 0.45;
+  cursor: not-allowed;
+}
+
+.chipActive {
+  background: var(--accent-coral, #e07256);
+  color: white;
+  border-color: var(--accent-coral, #e07256);
+  font-weight: 600;
+}
+
+.chipActive:active {
+  background: var(--accent-coral-dark, #c45a3f);
+}
+
+.miniLegend {
+  display: flex;
+  gap: 0.875rem;
+  font-size: 0.75rem;
+  color: var(--text-secondary, #5c564d);
+  margin-top: -0.25rem;
+}
+
+.miniLegend span {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.3rem;
+}
+
+.miniDot {
+  display: inline-block;
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+}
+
 @media (max-width: 768px) {
   .chartWrapper {
     font-size: 0.875rem;
   }
 }
+
+@media (max-width: 640px) {
+  .mobileChips {
+    display: block;
+  }
+  /* The desktop "click the legend to show progress" hint is irrelevant
+     once the chip row is the disclosure mechanism. */
+  .chartHint {
+    display: none;
+  }
+}