fix: Swap folder tree count with menu button on hover

The tree count and hamburger menu were overlapping at the right edge of
folder items. Now the count hides on hover and the menu button appears
in its place.

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

frontend/src/components/library/FolderSidebar.tsx

@@ -136,10 +136,10 @@ function FolderItem({
           )}
           <Folder className="h-4 w-4 shrink-0" style={{ color: folder.color }} />
           <span className="flex-1 truncate text-left">{folder.name}</span>
-          <span className="text-xs text-muted-foreground">{folder.tree_count}</span>
+          <span className="text-xs text-muted-foreground group-hover:hidden">{folder.tree_count}</span>
         </button>
 
-        {/* Folder menu button */}
+        {/* Folder menu button - replaces tree count on hover */}
         <button
           onClick={(e) => {
             e.stopPropagation()
@@ -147,7 +147,7 @@ function FolderItem({
           }}
           className={cn(
             'absolute right-1 top-1/2 -translate-y-1/2 rounded p-1',
-            'opacity-0 group-hover:opacity-100',
+            'hidden group-hover:block',
             'hover:bg-accent'
           )}
         >