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/components/StackedNote.vue

@@ -278,7 +278,7 @@ $border-color: rgba(18, 19, 58, 0.2);
     section {
       padding: 1rem 0 2rem;
       overflow-x: auto;
-      overflow-y: hidden;
+      overflow-y: clip;
     }
 
     .note-content {