feat(admin): add LLM call logs, system info, plugin list pages

- ai-core/llm: ring-buffer call logger with stats
- Gateway: /api/v1/admin/llm-calls, /system, /plugins, /sessions endpoints
- Frontend: LLMCallsPage (table + stats), SystemPage (version/plugins/sessions)
- Admin nav expanded to 7 items

frontend/web/src/components/admin/AdminLayout.tsx

@@ -6,6 +6,8 @@ const navItems = [
   { to: '/admin/iot', label: 'IoT 设备', icon: '⌂' },
   { to: '/admin/plugins', label: '插件管理', icon: '◇' },
   { to: '/admin/memory', label: '记忆管理', icon: '♡' },
+  { to: '/admin/llm-calls', label: 'LLM 日志', icon: '🧠' },
+  { to: '/admin/system', label: '系统信息', icon: '⊞' },
 ]
 
 export default function AdminLayout() {