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/SchoolDetailView.tsx

@@ -106,6 +106,10 @@ export function SchoolDetailView({
     }
   };
 
+  const scrollToTop = () => {
+    if (typeof window !== 'undefined') window.scrollTo({ top: 0, behavior: 'smooth' });
+  };
+
   useEffect(() => {
     const el = sectionLinksRef.current;
     if (!el) return;
@@ -306,6 +310,13 @@ export function SchoolDetailView({
 
   return (
     <div className={styles.container}>
+      {/* Standalone back link, above the header — returns to wherever the
+          user came from. Scrolls away with the page (the sticky bar keeps a
+          "back to top" control in its place). */}
+      <button type="button" onClick={handleBack} className={styles.topBack}>
+        <span aria-hidden="true">←</span> Back
+      </button>
+
       {/* Header */}
       <header className={styles.header}>
         <div className={styles.headerContent}>
@@ -439,9 +450,9 @@ export function SchoolDetailView({
 
       {/* Sticky Section Navigation — docks under the global header */}
       <nav className={styles.sectionNav} aria-label="Page sections">
-        <button onClick={handleBack} className={styles.sectionNavBack} aria-label="Back">
-          <span aria-hidden="true">←</span>
-          <span className={styles.sectionNavBackLabel}>Back</span>
+        <button onClick={scrollToTop} className={styles.sectionNavBack} aria-label="Back to top">
+          <span aria-hidden="true">↑</span>
+          <span className={styles.sectionNavBackLabel}>Top</span>
         </button>
 
         {/* Desktop: scrolling section links */}