feat: Add Plugins dropdown menu to Business sidebar

- Added Plugins collapsible menu with 3 items:
  - Plugin Marketplace (/plugins/marketplace)
  - My Plugins (/plugins/my-plugins)
  - Plugin Docs (/help/plugins)
- Moved Plugin Docs from Help dropdown to Plugins dropdown
- Added Plug, ShoppingBag, Package icons from lucide-react
- Added translations for plugin navigation items
- Auto-opens dropdown when on /plugins/* routes

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

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

frontend/src/i18n/locales/en.json

@@ -94,7 +94,10 @@
     "ticketingHelp": "Ticketing System",
     "apiDocs": "API Docs",
     "pluginDocs": "Plugin Docs",
-    "contactSupport": "Contact Support"
+    "contactSupport": "Contact Support",
+    "plugins": "Plugins",
+    "pluginMarketplace": "Marketplace",
+    "myPlugins": "My Plugins"
   },
   "help": {
     "guide": {