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.module.css

@@ -4,6 +4,29 @@
     width: 100%;
 }
 
+/* Standalone back link, sits above the header card on the page background. */
+.topBack {
+    display: inline-flex;
+    align-items: center;
+    gap: 0.4rem;
+    margin: 0 0 0.75rem;
+    padding: 0.25rem 0;
+    font-size: 1.0625rem;
+    font-weight: 600;
+    color: var(--accent-coral, #e07256);
+    background: none;
+    border: none;
+    cursor: pointer;
+    line-height: 1.2;
+    transition: color 0.15s ease;
+}
+
+.topBack:hover {
+    color: var(--accent-coral-dark, #c85a3e);
+    text-decoration: underline;
+    text-underline-offset: 2px;
+}
+
 /* ── Header ──────────────────────────────────────────── */
 .header {
     background: var(--bg-card, white);