fix(layout): prevent document-level scroll-y when stacked notes overflow

Contain horizontal overflow within #main-app instead of leaking to the
document, which caused a horizontal scrollbar to consume viewport height
and trigger an unwanted vertical scrollbar. Also fix note pane height
to use 100% instead of 100vh, and switch useResizeContainer to minWidth
so the flex container can grow when the window is wider than the notes.
Add a window resize listener to keep the value accurate on resize.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/FluxNote.vue

@@ -204,7 +204,7 @@ $header-height: 40px;
     display: flex;
     flex-direction: column;
     overflow-y: auto;
-    height: 100vh;
+    height: 100%;
     position: sticky;
 
     &:not(:first-child) {