fix: force scrollbar visibility to prevent layout shift on tall pages

Add overflow-y: scroll to body so the scrollbar is always present,
preventing the ~15px layout shift when a page exceeds viewport height
(e.g., games with long descriptions). Also add overflow-x: hidden to
guard against any horizontal overflow breaking the layout.

frontend/src/app.css

@@ -61,6 +61,8 @@ body {
   color: var(--text);
   min-height: 100vh;
   line-height: 1.6;
+  overflow-y: scroll;
+  overflow-x: hidden;
 }
 
 a {