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/RankingsView.tsx

@@ -258,13 +258,13 @@ export function RankingsView({
                         <strong>{displayValue}</strong>
                       </td>
                       <td className={styles.actionCell}>
-                        <a href={`/school/${ranking.urn}`} className={styles.viewButton}>View</a>
+                        <a href={`/school/${ranking.urn}`} className="btn btn-tertiary btn-sm">View</a>
                         <button
                           onClick={() => handleAddToCompare(ranking)}
                           disabled={alreadyInComparison}
-                          className={styles.addButton}
+                          className={alreadyInComparison ? 'btn btn-active btn-sm' : 'btn btn-secondary btn-sm'}
                         >
-                          {alreadyInComparison ? '✓ Added' : '+ Add'}
+                          {alreadyInComparison ? '✓ Comparing' : '+ Compare'}
                         </button>
                       </td>
                     </tr>