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/lib/utils.ts

@@ -275,6 +275,27 @@ export function getChartColor(index: number): string {
   return CHART_COLORS[index % CHART_COLORS.length];
 }
 
+/**
+ * Darker counterparts of CHART_COLORS for use as TEXT on the site's light
+ * backgrounds — the raw series colours fail WCAG AA (e.g. 1.9:1 on cream).
+ * Same hue per index so text still keys to its chart line. All ≥4.5:1 on
+ * #f3ede4 and lighter.
+ */
+export const CHART_TEXT_COLORS = [
+  '#256868', // teal
+  '#c2255c', // pink/red
+  '#2069ad', // blue
+  '#8a5a00', // yellow → ochre
+  '#6d43c8', // purple
+  '#8f4e00', // orange → brown
+  '#5f5952', // grey
+  '#a300a3', // magenta
+];
+
+export function getChartTextColor(index: number): string {
+  return CHART_TEXT_COLORS[index % CHART_TEXT_COLORS.length];
+}
+
 /**
  * Convert RGB color to RGBA with opacity
  */