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/SchoolSearchModal.tsx

@@ -122,9 +122,9 @@ export function SchoolSearchModal({ isOpen, onClose }: SchoolSearchModalProps) {
                 <button
                   onClick={() => handleAddSchool(school)}
                   disabled={alreadySelected || !canAddMore}
-                  className={styles.addButton}
+                  className={alreadySelected ? 'btn btn-active' : 'btn btn-secondary'}
                 >
-                  {alreadySelected ? '✓ Added' : '+ Add'}
+                  {alreadySelected ? '✓ Comparing' : '+ Compare'}
                 </button>
               </div>
             );