feat(school-detail): page-wide improvements across 5 sections

History chart
- Strip the redundant school-name title (already the page heading)
- Default to 2 visible lines: Reading, Writing & Maths expected % (teal,
  bold) + Exceeding (gold, lighter); progress score lines hidden by
  default, togglable via legend
- Add dashed national average reference line for RWM (primary) or
  Attainment 8 (secondary) so the school's trajectory is always in
  context
- Add trend summary chip above the chart computed from the data
  ("↓ Peaked at 90% (2016/17), currently 70%")
- Add COVID footnote when 2019/20 and 2020/21 data is absent

Ofsted section
- Collapse the four identical "Outstanding / Outstanding / Outstanding /
  Outstanding" boxes into a single prose line when all sub-grades match
  the overall verdict; show individual cards only when grades differ

SATs sub-metrics
- DeltaChip vs national average on Expected level row for Reading,
  Writing and Maths (national averages already in the API response)

Admissions
- Fix label: "Year 3 places per year" → "Reception places per year" for
  primary schools

Pupils & Inclusion
- DeltaChip + national avg hint on Eligible for pupil premium and
  Pupils receiving SEN support (both keys present in /api/national-averages)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nextjs-app/components/PerformanceChart.module.css

@@ -1,9 +1,34 @@
+.chartOuter {
+  display: flex;
+  flex-direction: column;
+  gap: 0.75rem;
+}
+
+.trendSummary {
+  font-size: 0.875rem;
+  font-weight: 600;
+  color: var(--text-secondary, #5c564d);
+  padding: 0.5rem 0.875rem;
+  background: var(--bg-secondary, #f3ede4);
+  border-left: 3px solid var(--accent-teal, #2d7d7d);
+  border-radius: 0 4px 4px 0;
+  align-self: flex-start;
+}
+
 .chartWrapper {
   width: 100%;
   height: 100%;
   position: relative;
 }
 
+.covidNote,
+.chartHint {
+  font-size: 0.75rem;
+  color: var(--text-muted, #8a847a);
+  margin: 0;
+  font-style: italic;
+}
+
 @media (max-width: 768px) {
   .chartWrapper {
     font-size: 0.875rem;