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/styles/app.css

@@ -52,6 +52,7 @@
 html,
 body {
   height: 100dvh;
+  overflow: hidden;
   scroll-behavior: smooth;
 }