feat(ui): site-wide UX/UI audit — unified buttons, tokens, accessibility

- Add shared button system (.btn-primary/secondary/tertiary/active) in globals.css
- Replace 40+ hardcoded rgba() values with design tokens across all CSS modules
- Add skip link, :focus-visible indicators, and ARIA landmarks
- Standardise button labels ("+ Compare" / "✓ Comparing") across all components
- Add collapse/minimize toggle to ComparisonToast
- Fix heading hierarchy (h3→h2 in ComparisonView)
- Add aria-live on search results, aria-label on trend SVGs
- Add "Search" nav link, fix footer empty section, unify max-widths
- Darken --text-muted for WCAG AA compliance (4.6:1 contrast ratio)
- Net reduction of ~180 lines through button style deduplication

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

nextjs-app/components/SchoolRow.tsx

@@ -81,17 +81,17 @@ export function SchoolRow({
                   title={`Previous year: ${formatPercentage(school.prev_rwm_expected_pct)}`}
                 >
                   {trend === 'up' && (
-                    <svg viewBox="0 0 16 16" fill="none" width="9" height="9">
+                    <svg viewBox="0 0 16 16" fill="none" width="9" height="9" aria-label="Trend up">
                       <path d="M8 3L14 10H2L8 3Z" fill="currentColor" />
                     </svg>
                   )}
                   {trend === 'down' && (
-                    <svg viewBox="0 0 16 16" fill="none" width="9" height="9">
+                    <svg viewBox="0 0 16 16" fill="none" width="9" height="9" aria-label="Trend down">
                       <path d="M8 13L2 6H14L8 13Z" fill="currentColor" />
                     </svg>
                   )}
                   {trend === 'stable' && (
-                    <svg viewBox="0 0 16 16" fill="none" width="9" height="9">
+                    <svg viewBox="0 0 16 16" fill="none" width="9" height="9" aria-label="Trend stable">
                       <rect x="2" y="7" width="12" height="2" rx="1" fill="currentColor" />
                     </svg>
                   )}
@@ -144,15 +144,15 @@ export function SchoolRow({
 
       {/* Right: actions, vertically centred */}
       <div className={styles.rowActions}>
-        <a href={`/school/${school.urn}`} className={styles.btnView}>
+        <a href={`/school/${school.urn}`} className="btn btn-tertiary btn-sm">
           View
         </a>
         {(onAddToCompare || onRemoveFromCompare) && (
           <button
             onClick={handleCompareClick}
-            className={isInCompare ? styles.btnRemove : styles.btnCompare}
+            className={isInCompare ? 'btn btn-active btn-sm' : 'btn btn-secondary btn-sm'}
           >
-            {isInCompare ? '✓ Remove' : '+ Add'}
+            {isInCompare ? '✓ Comparing' : '+ Compare'}
           </button>
         )}
       </div>