feat(saas): Phase 3 桌面端 SaaS 集成 — 客户端、Store、UI、LLM 适配器

- saas-client.ts: SaaS HTTP 客户端 (登录/注册/Token/模型列表/Chat Relay/配置同步)
- saasStore.ts: Zustand 状态管理 (登录态、连接模式、可用模型、localStorage 持久化)
- connectionStore.ts: 集成 SaaS 模式分支 (connect() 优先检查 SaaS 连接模式)
- llm-service.ts: SaasLLMAdapter 实现 (通过 SaaS Relay 代理 LLM 调用)
- SaaSLogin.tsx: 登录/注册表单 (服务器地址、用户名、密码、邮箱)
- SaaSStatus.tsx: 连接状态展示 (账号信息、健康检查、可用模型列表)
- SaaSSettings.tsx: SaaS 设置页面入口 (登录态切换、功能列表)
- SettingsLayout.tsx: 添加 SaaS 平台菜单项
- store/index.ts: 导出 useSaaSStore

desktop/src/components/Settings/SettingsLayout.tsx

@@ -18,6 +18,7 @@ import {
   Heart,
   Key,
   Database,
+  Cloud,
 } from 'lucide-react';
 import { silentErrorHandler } from '../../lib/error-utils';
 import { General } from './General';
@@ -37,6 +38,7 @@ import { TaskList } from '../TaskList';
 import { HeartbeatConfig } from '../HeartbeatConfig';
 import { SecureStorage } from './SecureStorage';
 import { VikingPanel } from '../VikingPanel';
+import { SaaSSettings } from '../SaaS/SaaSSettings';
 
 interface SettingsLayoutProps {
   onBack: () => void;
@@ -54,6 +56,7 @@ type SettingsPage =
   | 'privacy'
   | 'security'
   | 'storage'
+  | 'saas'
   | 'viking'
   | 'audit'
   | 'tasks'
@@ -72,6 +75,7 @@ const menuItems: { id: SettingsPage; label: string; icon: React.ReactNode }[] =
   { id: 'workspace', label: '工作区', icon: <FolderOpen className="w-4 h-4" /> },
   { id: 'privacy', label: '数据与隐私', icon: <Shield className="w-4 h-4" /> },
   { id: 'storage', label: '安全存储', icon: <Key className="w-4 h-4" /> },
+  { id: 'saas', label: 'SaaS 平台', icon: <Cloud className="w-4 h-4" /> },
   { id: 'viking', label: '语义记忆', icon: <Database className="w-4 h-4" /> },
   { id: 'security', label: '安全状态', icon: <Shield className="w-4 h-4" /> },
   { id: 'audit', label: '审计日志', icon: <ClipboardList className="w-4 h-4" /> },
@@ -97,6 +101,7 @@ export function SettingsLayout({ onBack }: SettingsLayoutProps) {
       case 'workspace': return <Workspace />;
       case 'privacy': return <Privacy />;
       case 'storage': return <SecureStorage />;
+      case 'saas': return <SaaSSettings />;
       case 'security': return (
         <div className="space-y-6">
           <div>