fix(ui): move back button into sticky nav, fix sticky nav offset

- Remove standalone back button div (looked out of place)
- Back button now lives in the sticky section nav bar, styled as a
  bordered pill with coral accent — consistent with page design
- Fix sticky nav top offset from 0 to 3rem so it sticks below the
  site-wide header instead of sliding behind it
- Increase scroll-margin-top on cards to 6rem to account for both
  site header and section nav height

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

nextjs-app/components/SchoolDetailView.tsx

@@ -110,11 +110,6 @@ export function SchoolDetailView({
 
   return (
     <div className={styles.container}>
-      {/* Back Navigation */}
-      <div className={styles.backNav}>
-        <button onClick={() => router.back()} className={styles.backButton}>← Back</button>
-      </div>
-
       {/* Header */}
       <header className={styles.header}>
         <div className={styles.headerContent}>
@@ -174,15 +169,15 @@ export function SchoolDetailView({
       </header>
 
       {/* Sticky Section Navigation */}
-      {navItems.length > 0 && (
-        <nav className={styles.sectionNav} aria-label="Page sections">
-          <div className={styles.sectionNavInner}>
-            {navItems.map(({ id, label }) => (
-              <a key={id} href={`#${id}`} className={styles.sectionNavLink}>{label}</a>
-            ))}
-          </div>
-        </nav>
-      )}
+      <nav className={styles.sectionNav} aria-label="Page sections">
+        <div className={styles.sectionNavInner}>
+          <button onClick={() => router.back()} className={styles.sectionNavBack}>← Back</button>
+          {navItems.length > 0 && <div className={styles.sectionNavDivider} />}
+          {navItems.map(({ id, label }) => (
+            <a key={id} href={`#${id}`} className={styles.sectionNavLink}>{label}</a>
+          ))}
+        </div>
+      </nav>
 
       {/* Ofsted Rating */}
       {ofsted && (