feat(school-detail): move Back to a standalone link, dock "Back to top" in the nav

Replicate the Autotrader pattern: pull the page-back control out of the
sticky section-nav bar and place a standalone "← Back" text link above the
header card, on the page background, where it scrolls away with the page.
It keeps the context-aware behaviour (router.back → /search fallback); the
secondary view gains the same fallback for parity (it previously used a bare
router.back() that dead-ends on deep links).

The pinned slot the Back button vacated now holds a "↑ Top" control that
smooth-scrolls to the top, so the sticky bar keeps a useful affordance.
Applies to both primary and secondary detail views.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nextjs-app/components/SecondarySchoolDetailView.tsx

@@ -124,6 +124,20 @@ export function SecondarySchoolDetailView({
     }
   };
 
+  // Back returns wherever the user came from; deep-links fall back to search
+  // so the button never dead-ends or leaves the site.
+  const handleBack = () => {
+    if (typeof window !== 'undefined' && window.history.length > 1) {
+      router.back();
+    } else {
+      router.push('/search');
+    }
+  };
+
+  const scrollToTop = () => {
+    if (typeof window !== 'undefined') window.scrollTo({ top: 0, behavior: 'smooth' });
+  };
+
   useEffect(() => {
     track('school_viewed', {
       urn: schoolInfo.urn,
@@ -200,6 +214,12 @@ export function SecondarySchoolDetailView({
 
   return (
     <div className={styles.container}>
+      {/* Standalone back link, above the header — returns wherever the user
+          came from. Scrolls away; the sticky bar keeps a "back to top" control. */}
+      <button type="button" onClick={handleBack} className={styles.topBack}>
+        <span aria-hidden="true">←</span> Back
+      </button>
+
       {/* ── Header ─────────────────────────────────────── */}
       <header className={styles.header}>
         <div className={styles.headerContent}>
@@ -324,7 +344,7 @@ export function SecondarySchoolDetailView({
       {/* ── Sticky section navigation ─────────────────────── */}
       <nav className={styles.tabNav} aria-label="Page sections">
         <div className={styles.tabNavInner}>
-          <button onClick={() => router.back()} className={styles.backBtn}>← Back</button>
+          <button onClick={scrollToTop} className={styles.backBtn} aria-label="Back to top">↑ Top</button>
           {navItems.length > 0 && <div className={styles.tabNavDivider} />}
           {navItems.map(({ id, label }) => (
             <a