fix: prevent layout shift by using 100vw body instead of scrollbar-gutter

Setting body { width: 100vw; overflow-x: hidden; } means the body is
always the full viewport width regardless of scrollbar presence.
margin: 0 auto centers content within the body, which doesn't
change width when a vertical scrollbar appears — so the content
stays in place.

html { overflow-x: hidden; } clips any body overflow past the
viewport (body is 100vw while html content area shrinks with
scrollbar). No gutter discoloration since there's no gutter
reservation — the scrollbar appears naturally on the html element.

frontend/src/app.css

@@ -62,10 +62,11 @@ body {
   min-height: 100vh;
   line-height: 1.6;
   overflow-x: hidden;
+  width: 100vw;
 }
 
 html {
-  scrollbar-gutter: stable;
+  overflow-x: hidden;
 }
 
 a {