fix(documents): FolderTreeSidebar surfaces fetch error state

Folder query failures previously rendered identically to an empty
list, hiding network problems from the user. Add an isError branch
that shows "Failed to load folders." in destructive color.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/documents/folder-tree-sidebar.tsx

@@ -25,12 +25,8 @@ interface FolderTreeSidebarProps {
  * Designed for unlimited depth — only the top level renders by default
  * so deep trees don't blow out the page; reps drill in by expanding.
  */
-export function FolderTreeSidebar({
-  selectedFolderId,
-  onSelect,
-  footer,
-}: FolderTreeSidebarProps) {
-  const { data: tree = [], isLoading } = useDocumentFolders();
+export function FolderTreeSidebar({ selectedFolderId, onSelect, footer }: FolderTreeSidebarProps) {
+  const { data: tree = [], isLoading, isError } = useDocumentFolders();
 
   return (
     <aside className="w-full sm:w-60 shrink-0 border-r bg-muted/40 p-2">
@@ -54,6 +50,8 @@ export function FolderTreeSidebar({
       <div className="mt-3 space-y-0.5">
         {isLoading ? (
           <p className="px-2 text-xs text-muted-foreground">Loading…</p>
+        ) : isError ? (
+          <p className="px-2 text-xs text-destructive">Failed to load folders.</p>
         ) : tree.length === 0 ? (
           <p className="px-2 text-xs text-muted-foreground">No folders yet.</p>
         ) : (