feat(school-detail): icon-only Compare on phone heroes; unclutter map bottom edge

On ≤640px the floating Compare button becomes a 40px round glyph (+ / ✓,
aria-labelled) like the section-nav compare icon, instead of a full-width
text pill over the map. The OSM attribution moves to the map band's
top-left in preview so it no longer collides with the school name sliding
up under the fade; fullscreen keeps Leaflet's default bottom-right since
the zoom control occupies the top-left there.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

nextjs-app/components/SchoolDetailView.module.css

@@ -87,6 +87,36 @@
     background: #fff;
 }
 
+/* Full label by default; phones over the map get an icon-only button
+   (same compact treatment as the section-nav compare icon). */
+.btnCompareGlyph {
+    display: none;
+}
+
+@media (max-width: 640px) {
+    .headerHasMap .actions .btnCompareLabel {
+        display: none;
+    }
+
+    .headerHasMap .actions .btnCompareGlyph {
+        display: inline;
+    }
+
+    .headerHasMap .actions .btnAdd,
+    .headerHasMap .actions .btnRemove {
+        display: inline-flex;
+        align-items: center;
+        justify-content: center;
+        flex: none;
+        width: 40px;
+        height: 40px;
+        padding: 0;
+        border-radius: 999px;
+        font-size: 1.375rem;
+        line-height: 1;
+    }
+}
+
 /* Inline "View on map ↗" trigger next to the address. */
 .mapLink {
     border: none;