feat(school-detail): mobile section nav becomes a menu; Compare shrinks to an icon

Fixes the cramped mobile bar where Back, the carried Compare pill and All ▾ all
competed with the scrolling section links:

- On mobile the horizontal swipe strip collapses into a single "Section: <current> ▾"
  button that opens the existing jump-to-section sheet — no fragile horizontal
  swipe, and it doubles as a "you are here" indicator.
- The carried Compare CTA becomes a compact 38px icon on mobile (compare-arrows +
  "add" badge; flips to a teal check when in the comparison), so it no longer
  crowds the section control. Desktop keeps the labelled pill.
- Back is icon-only on mobile (label hidden), text on desktop.
- Desktop is unchanged: horizontal links + All ▾, full CTA in the hero.

Both the mobile section menu and the desktop All ▾ open the same sheet.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lh7Js5xSetKNzLVr9ArXLF

nextjs-app/components/SchoolDetailView.module.css

@@ -205,6 +205,7 @@
     flex: none;
     display: inline-flex;
     align-items: center;
+    gap: 0.3rem;
     padding: 0.3rem 0.625rem;
     font-size: 0.75rem;
     font-weight: 600;
@@ -294,6 +295,119 @@
     color: white;
 }
 
+/* ── Mobile: the scrolling links collapse into one "section" menu button ──
+   (hidden on desktop, where the links fit). */
+.sectionNavMenu {
+    display: none; /* shown only ≤640px */
+    flex: 1;
+    min-width: 0;
+    align-items: center;
+    justify-content: space-between;
+    gap: 0.5rem;
+    min-height: 38px;
+    padding: 0.34rem 0.7rem;
+    background: var(--bg-secondary, #f3ede4);
+    border: 1px solid var(--border-color, #e5dfd5);
+    border-radius: 8px;
+    cursor: pointer;
+    font-family: var(--font-dm-sans), "DM Sans", sans-serif;
+    color: var(--text-primary, #1a1612);
+}
+
+.sectionNavMenuCur {
+    display: flex;
+    align-items: center;
+    gap: 0.45rem;
+    min-width: 0;
+}
+
+.sectionNavMenuEyebrow {
+    flex: none;
+    font-size: 0.64rem;
+    font-weight: 600;
+    letter-spacing: 0.04em;
+    text-transform: uppercase;
+    color: var(--text-muted, #6d685f);
+}
+
+.sectionNavMenuNow {
+    font-size: 0.85rem;
+    font-weight: 600;
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+}
+
+.sectionNavMenuChev {
+    flex: none;
+    color: var(--text-muted, #6d685f);
+    font-size: 0.7rem;
+}
+
+/* Compact icon version of the Compare CTA, used on mobile. */
+.sectionNavCompareIcon {
+    display: none; /* shown only ≤640px */
+    position: relative;
+    flex: none;
+    align-items: center;
+    justify-content: center;
+    width: 38px;
+    height: 38px;
+    border-radius: 9px;
+    border: 1px solid var(--accent-coral, #e07256);
+    background: var(--accent-coral, #e07256);
+    color: white;
+    cursor: pointer;
+    transition: background 0.15s ease, border-color 0.15s ease;
+}
+
+.sectionNavCompareIcon svg {
+    width: 19px;
+    height: 19px;
+}
+
+.sectionNavCompareBadge {
+    position: absolute;
+    top: -5px;
+    right: -5px;
+    width: 16px;
+    height: 16px;
+    border-radius: 50%;
+    background: var(--bg-card, white);
+    color: var(--accent-coral-dark, #c45a3f);
+    border: 1.5px solid var(--accent-coral, #e07256);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    font-size: 0.7rem;
+    font-weight: 800;
+    line-height: 1;
+}
+
+.sectionNavCompareIconIn {
+    background: var(--bg-card, white);
+    border-color: var(--accent-teal, #2d7d7d);
+    color: var(--accent-teal, #2d7d7d);
+}
+
+/* Swap which controls show at the mobile breakpoint. */
+@media (max-width: 640px) {
+    .sectionNavLinks,
+    .sectionNavCompare,
+    .sectionNavAll,
+    .sectionNavBackLabel {
+        display: none;
+    }
+
+    .sectionNavMenu {
+        display: flex;
+    }
+
+    .sectionNavCompareIcon {
+        display: inline-flex;
+    }
+}
+
 /* Compare CTA carried into the bar once the hero's button scrolls away. */
 .sectionNavCompare {
     flex: none;
@@ -333,13 +447,9 @@
     }
 }
 
-/* "All ▾" jump menu. */
-.sectionNavAllWrap {
-    flex: none;
-    position: relative;
-}
-
+/* "All ▾" jump menu (desktop). */
 .sectionNavAll {
+    flex: none;
     display: inline-flex;
     align-items: center;
     gap: 0.25rem;