fix: increase drawer close delay to 400ms for usability

120ms was too fast — mouse couldn't traverse the gap between
rail icon and drawer panel without triggering close.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/Sidebar.tsx

@@ -178,7 +178,7 @@ export function Sidebar() {
   }
 
   const closeFlyout = () => {
-    flyoutTimeout.current = setTimeout(() => setFlyoutIndex(null), 120)
+    flyoutTimeout.current = setTimeout(() => setFlyoutIndex(null), 400)
   }
 
   const keepFlyout = () => {