fix(Navbar): replace IntersectionObserver with scroll-based active section

app/components/Navbar.tsx

@@ -48,30 +48,22 @@ export function Navbar() {
   }, []);
 
   useEffect(() => {
-    const visible = new Set<string>();
-
-    const updateActive = () => {
+    const onScroll = () => {
+      // trigger line = 1/3 from top of viewport (= 2/3 from bottom)
+      const triggerY = window.scrollY + NAVBAR_HEIGHT + (window.innerHeight - NAVBAR_HEIGHT) / 3;
+      let current = "";
       for (const id of anchorIds) {
-        if (visible.has(id)) { setActiveSection(id); return; }
+        const el = document.getElementById(id);
+        if (!el) continue;
+        const top = el.getBoundingClientRect().top + window.scrollY;
+        if (top <= triggerY) current = id;
       }
-      setActiveSection("");
+      setActiveSection(current);
     };
 
-    const observers = anchorIds.map((id) => {
-      const el = document.getElementById(id);
-      if (!el) return null;
-      const obs = new IntersectionObserver(
-        ([entry]) => {
-          entry.isIntersecting ? visible.add(id) : visible.delete(id);
-          updateActive();
-        },
-        { rootMargin: "-100px 0px -67% 0px" }
-      );
-      obs.observe(el);
-      return obs;
-    });
-
-    return () => observers.forEach((o) => o?.disconnect());
+    onScroll();
+    window.addEventListener("scroll", onScroll, { passive: true });
+    return () => window.removeEventListener("scroll", onScroll);
   }, []);
 
   return (