fix(mobile): restore body scroll and prevent spurious section scroll

Three layered fixes for mobile note scrolling:

1. app.css / App.vue: on mobile, override overflow:hidden on html/body
   and overflow:visible on #main-app so content from useResizeContainer
   (which sets the note-container height to (n+1)*100vh) propagates to
   the document and document.body.scrollTop works again.

2. FluxNote.vue: give each .note an explicit height:100dvh on mobile so
   the percentage-based height:100% does not resolve against the
   inflated container height set by useResizeContainer.

3. StackedNote / StackedPublicNote: replace overflow-y:hidden with
   overflow-y:clip on the section. Unlike hidden, clip does not create a
   scroll container, so touch events fall through to the page scroll and
   the section never feels "draggable" when content fits within the note.

src/styles/app.css

@@ -56,6 +56,13 @@ body {
   scroll-behavior: smooth;
 }
 
+@media screen and (max-width: 768px) {
+  html,
+  body {
+    overflow-y: auto;
+  }
+}
+
 .columns {
   margin-left: 0;
   margin-right: 0;