feat(documents): folder tree + actions UI for system-managed folders

FolderTreeSidebar shows a lock marker on system_managed rows and renders
archived entity folders muted. FolderActionsMenu disables Rename /
Move / Delete with a tooltip explanation when a system folder is
selected; the server-side guard (Task 4) is the authoritative
rejection — the UI is the friendly first line.

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

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

@@ -1,7 +1,7 @@
 'use client';
 
 import { useState } from 'react';
-import { ChevronRight, Folder, FolderOpen, Inbox } from 'lucide-react';
+import { ChevronRight, Folder, FolderOpen, Inbox, Lock } from 'lucide-react';
 
 import { Button } from '@/components/ui/button';
 import { cn } from '@/lib/utils';
@@ -135,7 +135,10 @@ function FolderRow({
         <button
           type="button"
           onClick={() => onSelect(node.id)}
-          className="flex flex-1 items-center gap-1.5 truncate text-left"
+          className={cn(
+            'flex flex-1 items-center gap-1.5 truncate text-left',
+            node.archivedAt != null && 'text-muted-foreground',
+          )}
         >
           {open && hasChildren ? (
             <FolderOpen className="h-4 w-4 shrink-0" />
@@ -143,6 +146,9 @@ function FolderRow({
             <Folder className="h-4 w-4 shrink-0" />
           )}
           <span className="truncate">{node.name}</span>
+          {node.systemManaged ? (
+            <Lock className="ml-1 h-3 w-3 shrink-0 text-muted-foreground" aria-label="System folder" />
+          ) : null}
         </button>
       </div>
       {open