feat(detail): replace SATs text tables with cascade bar charts, add admissions bar and history accordion

Redesign the School Details page for better parent comprehension:
- New SatsChart component: horizontal cascade bars with ruler scale and
  national average marker (teal/coral palette matching site theme)
- Admissions section: visual progress bar showing 1st-preference demand
  vs available places, colour-coded by oversubscription status
- Historical data: collapse raw year-by-year table behind a disclosure
  element while keeping the performance line chart always visible
- EAL metric: add national average comparison via DeltaChip (backend now
  includes eal_pct in national averages endpoint)
- New formatWithSuppression utility for null/suppressed data handling

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

nextjs-app/lib/utils.ts

@@ -71,6 +71,13 @@ export function formatPercentage(value: number | null | undefined, decimals: num
   return `${value.toFixed(decimals)}%`;
 }
 
+export function formatWithSuppression(value: number | null | undefined): { display: string; suppressed: boolean } {
+  if (value == null) return { display: '—', suppressed: true };
+  return { display: formatPercentage(value), suppressed: false };
+}
+
+export const SUPPRESSED_TOOLTIP = 'Data not available — may be suppressed to protect small cohorts.';
+
 /**
  * Format a progress score (can be negative)
  */