feat(school-detail): highlight active section in sticky nav on scroll

Uses IntersectionObserver on each section element. Multiple thresholds
(0, 0.1, 0.25, 0.5, 0.75, 1.0) track the intersection ratio of every
section simultaneously; whichever has the highest visible ratio at any
moment becomes the active item. rootMargin offsets for the sticky nav
height so a section is only considered active once it's genuinely in
view beneath the bar.

Active link gets .sectionNavLinkActive — coral background + white text,
matching the phase tab active style used elsewhere in the product.
Observer is cleaned up on unmount.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nextjs-app/components/SchoolDetailView.module.css

@@ -196,6 +196,17 @@
   color: var(--text-primary, #1a1612);
 }
 
+.sectionNavLinkActive {
+  background: var(--accent-coral, #e07256);
+  color: white;
+  font-weight: 600;
+}
+
+.sectionNavLinkActive:hover {
+  background: var(--accent-coral-dark, #c45a3f);
+  color: white;
+}
+
 /* Unified card for all content sections */
 .card {
   background: var(--bg-card, white);