Add auto-collapse on mouse leave for toggle sidebar mode

In toggle mode, sidebar now collapses automatically when mouse
leaves the sidebar area (after being expanded by click).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/Sidebar.tsx

@@ -101,6 +101,10 @@ export default function Sidebar() {
   const handleMouseLeave = () => {
     hoverTimeoutRef.current = setTimeout(() => {
       setIsHovered(false);
+      // In toggle mode, collapse sidebar when mouse leaves (if expanded)
+      if (sidebarMode === 'toggle' && !isCollapsed && !isMobileOpen) {
+        toggleCollapse();
+      }
     }, 100);
   };