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>

backend/app.py

@@ -683,7 +683,7 @@ async def get_national_averages(request: Request):
         "reading_avg_score", "maths_avg_score", "gps_avg_score",
         "reading_progress", "writing_progress", "maths_progress",
         "overall_absence_pct", "persistent_absence_pct",
-        "disadvantaged_gap", "disadvantaged_pct", "sen_support_pct",
+        "disadvantaged_gap", "disadvantaged_pct", "sen_support_pct", "eal_pct",
     ]
     ks4_metrics = [
         "attainment_8_score", "progress_8_score",