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/Navigation.tsx

@@ -35,7 +35,13 @@ export function Navigation() {
           <span className={styles.logoText}>SchoolCompare</span>
         </Link>
 
-        <nav className={styles.nav}>
+        <nav className={styles.nav} aria-label="Main navigation">
+          <Link
+            href="/"
+            className={isActive('/') ? `${styles.navLink} ${styles.active}` : styles.navLink}
+          >
+            Search
+          </Link>
           <Link
             href="/compare"
             className={isActive('/compare') ? `${styles.navLink} ${styles.active}` : styles.navLink}