refactor(notes): restore fixed mobile heights for scroll math

Re-pin .note and .stacked-note to 100dvh on mobile and bring back the
container height in useResizeContainer so (index + 1) * height has a
reachable scroll target. Switch the polled scroll helper to that same
formula instead of offsetTop.

src/components/StackedNote.vue

@@ -313,6 +313,7 @@ $border-color: rgba(18, 19, 58, 0.2);
 @media screen and (max-width: 768px) {
   .stacked-note {
     padding: 0 0.75rem 1rem;
+    height: 100dvh;
 
     section {
       padding: 1rem 0 2rem;