nav: stop tabs sticking lit on Projects/Blog pages

On pages with no scroll-spy sections, activeIdx stays -1 and indexOf(link)
is also -1, so the pointerleave 'keep lit if active' check matched every
hovered tab. Guard with activeIdx >= 0 so a hovered tab always retracts when
nothing is active.

src/scripts/anim.ts

@@ -118,8 +118,12 @@ function initNav(): void {
 
   for (const a of links) {
     a.addEventListener("pointerenter", () => under(a, true));
+    // On leave, keep the underline only if this link IS the active scroll-spy
+    // section. Guard on activeIdx >= 0 so that when nothing is active — e.g. the
+    // Projects/Blog pages, where none of these sections exist and activeIdx stays
+    // -1 — a hovered tab doesn't stick lit via the -1 === indexOf(non-member) -1.
     a.addEventListener("pointerleave", () =>
-      under(a, sectioned.indexOf(a) === activeIdx),
+      under(a, activeIdx >= 0 && sectioned.indexOf(a) === activeIdx),
     );
   }