Add in-app wiki help system with accounts and members articles

Markdown-based help pages rendered in the admin UI. Sidebar category
navigation with search. Articles: Getting Started, Accounts Overview,
Members Overview, Payment Methods, Tax Exemptions. Written for
non-technical store staff.

packages/admin/src/routes/_authenticated.tsx

@@ -14,7 +14,7 @@ import {
   DropdownMenuSubTrigger,
   DropdownMenuSubContent,
 } from '@/components/ui/dropdown-menu'
-import { Users, UserRound, Sun, Moon, Monitor, LogOut, User, Palette } from 'lucide-react'
+import { Users, UserRound, HelpCircle, Sun, Moon, Monitor, LogOut, User, Palette } from 'lucide-react'
 
 export const Route = createFileRoute('/_authenticated')({
   beforeLoad: () => {
@@ -69,6 +69,14 @@ function AuthenticatedLayout() {
               <UserRound className="h-4 w-4" />
               Members
             </Link>
+            <Link
+              to="/help"
+              className="flex items-center gap-2 px-3 py-2 rounded-md text-sm text-sidebar-foreground hover:bg-sidebar-accent"
+              activeProps={{ className: 'flex items-center gap-2 px-3 py-2 rounded-md text-sm bg-sidebar-accent text-sidebar-accent-foreground' }}
+            >
+              <HelpCircle className="h-4 w-4" />
+              Help
+            </Link>
           </div>
 
           <div className="p-3 border-t border-sidebar-border">