fix: use scrollbar-gutter to prevent layout shift without always showing scrollbar

Replaces overflow-y: scroll with scrollbar-gutter: stable on html.
This reserves the scrollbar gutter space in the layout at all times,
so the content width never changes when a scrollbar appears/disappears.
The scrollbar itself only shows when content overflows (as desired).
Works on all modern browsers (Chrome 94+, Firefox 97+, Safari 15.4+).

frontend/src/app.css

@@ -61,10 +61,13 @@ body {
   color: var(--text);
   min-height: 100vh;
   line-height: 1.6;
-  overflow-y: scroll;
   overflow-x: hidden;
 }
 
+html {
+  scrollbar-gutter: stable;
+}
+
 a {
   color: var(--phosphor);
   text-decoration: none;