refactor: Remove Marketplace link from Plugins dropdown

Users can access the marketplace via the "Browse Marketplace" button
on the My Plugins page, so the redundant sidebar link has been removed.

The Plugins dropdown now contains:
- My Plugins
- Plugin Docs

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

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

frontend/src/components/Sidebar.tsx

@@ -20,7 +20,6 @@ import {
   LifeBuoy,
   Zap,
   Plug,
-  ShoppingBag,
   Package
 } from 'lucide-react';
 import { Business, User } from '../types';
@@ -204,14 +203,6 @@ const Sidebar: React.FC<SidebarProps> = ({ business, user, isCollapsed, toggleCo
               </button>
               {isPluginsOpen && !isCollapsed && (
                 <div className="ml-4 mt-1 space-y-1 border-l border-white/20 pl-4">
-                  <Link
-                    to="/plugins/marketplace"
-                    className={`flex items-center gap-3 py-2 text-sm font-medium rounded-lg transition-colors px-3 ${location.pathname === '/plugins/marketplace' ? 'bg-white/10 text-white' : 'text-white/60 hover:text-white hover:bg-white/5'}`}
-                    title={t('nav.pluginMarketplace', 'Marketplace')}
-                  >
-                    <ShoppingBag size={16} className="shrink-0" />
-                    <span>{t('nav.pluginMarketplace', 'Marketplace')}</span>
-                  </Link>
                   <Link
                     to="/plugins/my-plugins"
                     className={`flex items-center gap-3 py-2 text-sm font-medium rounded-lg transition-colors px-3 ${location.pathname === '/plugins/my-plugins' ? 'bg-white/10 text-white' : 'text-white/60 hover:text-white hover:bg-white/5'}`}