fix(a11y): WCAG AA colour contrast across every route (P0.1)

Live axe-core inventory found 13 failing fg/bg pairs (3.12:1 coral text/
fills, 4.0-4.5 teal near-misses, 2.7 gold badge, 1.9 chart-colour text).
All replacement values validated ≥4.5:1 against every background they
sit on:

- --accent-coral-dark deepened to #b04a2e and used for coral text roles
  (nav active, back/map links, kickers, chips) and coral fills under
  white text (btn-primary, phase tabs, section-nav compare, chart chips);
  new --accent-coral-darker #9c3f26 for their hovers. Decorative coral
  (pins, bars, borders, focus ring) keeps the brand #e07256.
- --accent-teal darkened to #296f6f (rankings values, metric rows,
  ofsted grades, eyebrows all pass).
- Ofsted green #3c8c3c→#2f7a2f; off-token gold #b8920e→--accent-gold-text.
- Gender-split pink text #b45778→#a04a68; admissions tip numerals to a
  3.8:1 muted brown (28px display text).
- New CHART_TEXT_COLORS: AA-dark counterparts of the Chart.js series
  palette for compare-card values (swatch dots keep true series colour).

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

nextjs-app/components/LeafletMapInner.tsx

@@ -49,8 +49,8 @@ function buildPopupBadge(school: School): PopupBadge {
     const labels: Record<number, string> = { 1: 'Outstanding', 2: 'Good', 3: 'Req. Improvement', 4: 'Inadequate' };
     const colours: Record<number, string> = {
       1: 'background:#d4f0ea;color:#2d7d7d',
-      2: 'background:rgba(60,140,60,0.12);color:#3c8c3c',
-      3: 'background:#fef3cd;color:#b8920e',
+      2: 'background:rgba(60,140,60,0.12);color:#2f7a2f',
+      3: 'background:#fef3cd;color:#7a6800',
       4: 'background:#fde8e0;color:#e07256',
     };
     return { label: `${labels[school.ofsted_grade]}${yearStr}`, style: colours[school.ofsted_grade] };