feat(design): adopt the Cohort identity — new palette, type, mark and dark theme

Implements the direction agreed from the identity board: Route C ("Cohort")
with the paper ground from C1, the Schibsted Grotesk / Literata pairing from
C2, and the iris accent from C3. Dark theme is in scope from the start rather
than retrofitted.

The audit found three things wrong beyond taste:

* No brand asset set. og:image was absent entirely, so every link shared into
  a class WhatsApp group rendered as a bare grey card. apple-touch-icon pointed
  at an SVG, which iOS ignores, and the manifest shipped no PNGs, so Android
  installs had no icon. The header mark and the favicon had also drifted into
  two different logos.
* No colour discipline. --primary and --trend-down were the same coral, so the
  main CTA and "below average" shared a hue. 58 distinct hex values were spread
  across component CSS, and the chart palette was still Chart.js's stock demo
  colours.
* A dark theme that was declared but never built — themeColor announced a dark
  variant with no dark styling behind it.

What changed:

Colour now has exactly three jobs that never borrow each other's hues: brand
(iris) for interactive and identity, status (teal/amber) for above/below a
comparison point, and phase for categories. Teal/amber rather than green/red
keeps the above/below signal readable for every form of colour blindness.
Every chromatic literal in component CSS is now a token, and the JS-painted
surfaces (Chart.js, Leaflet) read the tokens through lib/theme so they follow
the theme instead of ignoring it.

The mark is the five-bar cohort spread — the same object as the distribution
strip inside a school row, built from opacity steps so it inverts cleanly.
components/Logo.tsx is the single source; the favicon, apple-icon and share
card all derive from its geometry.

globals.css drops 123 dead global classes left over from the vanilla-JS app
(only the btn family, .skip-link and .main were still referenced), along with
the noise overlay. It also gains prefers-reduced-motion support, which was
missing entirely, and a type scale so the 54 ad-hoc font sizes have somewhere
to converge.

Verified: tsc clean, 159 unit tests pass, production build succeeds and
prerenders /icon.svg, /apple-icon and /opengraph-image. Three e2e journeys
added for the asset set, the themeColor/background match, and the dark theme
actually repainting — all silent failures that nothing on the page reveals.

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

nextjs-app/components/LeafletMapInner.tsx

@@ -48,21 +48,21 @@ function buildPopupBadge(school: School): PopupBadge {
   // ofsted_framework is the raw event grouping, never "ReportCard").
   if (school.ofsted_rc_date) {
     const rcYear = new Date(school.ofsted_rc_date).getFullYear();
-    return { label: `Report Card · ${rcYear}`, style: 'background:#5a3a6e;color:#fff' };
+    return { label: `Report Card · ${rcYear}`, style: 'background:var(--phase-secondary-text);color:var(--text-inverse)' };
   }
   const year = school.ofsted_date ? new Date(school.ofsted_date).getFullYear() : null;
   const yearStr = year ? ` · ${year}` : '';
   if (school.ofsted_grade) {
     const labels: Record<number, string> = { 1: 'Outstanding', 2: 'Good', 3: 'Req. Improvement', 4: 'Inadequate' };
     const colours: Record<number, string> = {
-      1: 'background:#d4f0ea;color:#2d7d7d',
-      2: 'background:rgba(60,140,60,0.12);color:#2f7a2f',
-      3: 'background:#fef3cd;color:#7a6800',
-      4: 'background:#fde8e0;color:#e07256',
+      1: 'background:var(--status-above-bg);color:var(--status-above)',
+      2: 'background:var(--status-above-bg);color:var(--status-above)',
+      3: 'background:var(--status-below-bg);color:var(--status-below)',
+      4: 'background:var(--brand-bg);color:var(--brand)',
     };
     return { label: `${labels[school.ofsted_grade]}${yearStr}`, style: colours[school.ofsted_grade] };
   }
-  return { label: 'Not yet inspected', style: 'background:#e0e0e0;color:#666' };
+  return { label: 'Not yet inspected', style: 'background:var(--bg-secondary);color:var(--text-muted)' };
 }
 
 export default function LeafletMapInner({ schools, center, zoom, referencePoint, onMarkerClick, nationalAvgRwm, laAverages }: LeafletMapInnerProps) {
@@ -99,7 +99,7 @@ export default function LeafletMapInner({ schools, center, zoom, referencePoint,
       const refIcon = L.divIcon({
         html: `<div style="
           width: 20px; height: 20px;
-          background: #e07256;
+          background: var(--brand);
           border: 3px solid white;
           border-radius: 50%;
           box-shadow: 0 2px 8px rgba(0,0,0,0.35);
@@ -145,13 +145,13 @@ export default function LeafletMapInner({ schools, center, zoom, referencePoint,
             const sign = diff >= 0 ? '+' : '';
             // Att8 scores range 0–90 in 0.1 increments; ±0.5 is meaningful here
             // vs primary RWM % where ±2 pts is the threshold
-            const colour = diff >= 0.5 ? '#2d7d7d' : diff <= -0.5 ? '#e07256' : '#8a847a';
+            const colour = diff >= 0.5 ? 'var(--status-above)' : diff <= -0.5 ? 'var(--brand)' : 'var(--text-muted)';
             const laName = escapeHtml(school.local_authority ?? 'LA');
             deltaLine = `<div style="font-size:11px;font-weight:600;color:${colour}">${sign}${diff} vs ${laName} avg</div>`;
           }
           metricHtml = `<div style="margin-bottom:4px">
-  <span style="font-size:20px;font-weight:700;color:#1a1612;font-family:Georgia,serif">${score.toFixed(1)}</span>
-  <span style="font-size:11px;color:#8a847a;margin-left:4px">Attainment 8</span>
+  <span style="font-size:20px;font-weight:700;color:var(--text-primary);font-family:Georgia,serif">${score.toFixed(1)}</span>
+  <span style="font-size:11px;color:var(--text-muted);margin-left:4px">Attainment 8</span>
   ${deltaLine}
 </div>`;
         } else if (school.rwm_expected_pct != null) {
@@ -159,7 +159,7 @@ export default function LeafletMapInner({ schools, center, zoom, referencePoint,
           let deltaLine = '';
           if (nationalAvgRwm != null) {
             const diff = Math.round(rwm - nationalAvgRwm);
-            const colour = diff >= 2 ? '#2d7d7d' : diff <= -2 ? '#e07256' : '#8a847a';
+            const colour = diff >= 2 ? 'var(--status-above)' : diff <= -2 ? 'var(--brand)' : 'var(--text-muted)';
             const text =
               diff >= 2 ? `+${diff} pts vs national` :
               diff <= -2 ? `${diff} pts vs national` :
@@ -167,8 +167,8 @@ export default function LeafletMapInner({ schools, center, zoom, referencePoint,
             deltaLine = `<div style="font-size:11px;font-weight:600;color:${colour}">${text}</div>`;
           }
           metricHtml = `<div style="margin-bottom:4px">
-  <span style="font-size:20px;font-weight:700;color:#1a1612;font-family:Georgia,serif">${rwm}%</span>
-  <span style="font-size:11px;color:#8a847a;margin-left:4px">Reading, Writing &amp; Maths</span>
+  <span style="font-size:20px;font-weight:700;color:var(--text-primary);font-family:Georgia,serif">${rwm}%</span>
+  <span style="font-size:11px;color:var(--text-muted);margin-left:4px">Reading, Writing &amp; Maths</span>
   ${deltaLine}
 </div>`;
         }
@@ -177,14 +177,14 @@ export default function LeafletMapInner({ schools, center, zoom, referencePoint,
 
         const popupContent = `<div style="font-family:system-ui,sans-serif;min-width:240px;max-width:280px;padding:0">
   <div style="display:flex;justify-content:space-between;align-items:flex-start;gap:8px;margin-bottom:6px">
-    <strong style="font-size:13px;color:#1a1612;line-height:1.3">${escapeHtml(school.school_name)}</strong>
+    <strong style="font-size:13px;color:var(--text-primary);line-height:1.3">${escapeHtml(school.school_name)}</strong>
     <span style="font-size:10px;font-weight:700;padding:2px 6px;border-radius:3px;white-space:nowrap;flex-shrink:0;${badge.style}">${badge.label}</span>
   </div>
-  <div style="font-size:11px;color:#8a847a;margin-bottom:8px">
+  <div style="font-size:11px;color:var(--text-muted);margin-bottom:8px">
     ${phaseLabel}${school.local_authority ? ` · ${escapeHtml(school.local_authority)}` : ''}${distanceStr}
   </div>
   ${metricHtml}
-  <a href="${slug}" style="display:block;text-align:center;padding:6px;background:#2d7d7d;color:white;border-radius:5px;text-decoration:none;font-size:12px;font-weight:600;margin-top:8px">View Details →</a>
+  <a href="${slug}" style="display:block;text-align:center;padding:6px;background:var(--status-above);color:white;border-radius:5px;text-decoration:none;font-size:12px;font-weight:600;margin-top:8px">View Details →</a>
 </div>`;
 
         marker.bindPopup(popupContent);