feat(ui): auto-hide sidebar scrollbar when not hovering

Replace scrollbar-none with a scrollbar-auto-hide utility that keeps the
scrollbar thumb transparent by default and reveals it on container hover.

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

ui/src/components/Sidebar.tsx

@@ -65,7 +65,7 @@ export function Sidebar() {
         </Button>
       </div>
 
-      <nav className="flex-1 min-h-0 overflow-y-auto scrollbar-none flex flex-col gap-4 px-3 py-2">
+      <nav className="flex-1 min-h-0 overflow-y-auto scrollbar-auto-hide flex flex-col gap-4 px-3 py-2">
         <div className="flex flex-col gap-0.5">
           {/* New Issue button aligned with nav items */}
           <button