polish(school-detail): tighten hero layout and drop redundant chip

- Drop the "Above national average" chip — the DeltaChip under the 70% /
  Attainment 8 number already carries the same signal, so the chip was
  duplicative and added noise.
- At-a-glance stats: switch from grid(auto-fit) to flex with a fixed
  3rem column gap so the numbers cluster at the start of the row rather
  than spreading across the full width of the header card.
- Add to Compare button: larger padding, bumped font, soft shadow, and
  self-align centre so it sits in balance with the bigger headline
  rather than floating tiny in the top-right corner.

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

nextjs-app/components/SchoolDetailView.tsx

@@ -232,23 +232,6 @@ export function SchoolDetailView({
             </div>
           )}
 
-          {isPrimary && heroRwm != null && heroRwmNat != null && heroRwm > heroRwmNat && (
-            <div className={`${styles.heroChip} ${styles['tone-teal']}`}>
-              <div className={styles.heroChipTitle}>Above national average</div>
-              <div className={styles.heroChipSub}>
-                Reading, Writing &amp; Maths · {Math.round(heroRwm)} vs {Math.round(heroRwmNat)}
-              </div>
-            </div>
-          )}
-
-          {isSecondary && heroAtt8 != null && heroAtt8Nat != null && heroAtt8 > heroAtt8Nat && (
-            <div className={`${styles.heroChip} ${styles['tone-teal']}`}>
-              <div className={styles.heroChipTitle}>Above national average</div>
-              <div className={styles.heroChipSub}>
-                Attainment 8 · {heroAtt8.toFixed(1)} vs {heroAtt8Nat.toFixed(1)}
-              </div>
-            </div>
-          )}
         </div>
 
         {/* At-a-glance stats row */}