feat(desktop): add billing frontend — plans, subscription, payment flow

Sprint 1: Desktop 计费闭环

- Add 7 billing types to saas-types.ts (BillingPlan, Subscription, UsageQuota, etc.)
- Add 6 billing API methods to saas-billing.ts (listPlans, getSubscription, createPayment, etc.)
- Extend saas-client.ts with interface merging for billing methods
- Extend saasStore with billing state/actions (plans, subscription, payment polling)
- Create PricingPage component with plan cards, usage bars, and checkout modal
- Add billing page entry in SettingsLayout (CreditCard icon + route)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src/components/Settings/SettingsLayout.tsx

@@ -19,6 +19,7 @@ import {
   Key,
   Database,
   Cloud,
+  CreditCard,
 } from 'lucide-react';
 import { silentErrorHandler } from '../../lib/error-utils';
 import { General } from './General';
@@ -39,6 +40,7 @@ import { HeartbeatConfig } from '../HeartbeatConfig';
 import { SecureStorage } from './SecureStorage';
 import { VikingPanel } from '../VikingPanel';
 import { SaaSSettings } from '../SaaS/SaaSSettings';
+import { PricingPage } from '../SaaS/PricingPage';
 
 interface SettingsLayoutProps {
   onBack: () => void;
@@ -57,6 +59,7 @@ type SettingsPage =
   | 'security'
   | 'storage'
   | 'saas'
+  | 'billing'
   | 'viking'
   | 'audit'
   | 'tasks'
@@ -76,6 +79,7 @@ const menuItems: { id: SettingsPage; label: string; icon: React.ReactNode }[] =
   { 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: 'billing', label: '订阅与计费', icon: <CreditCard 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" /> },
@@ -102,6 +106,7 @@ export function SettingsLayout({ onBack }: SettingsLayoutProps) {
       case 'privacy': return <Privacy />;
       case 'storage': return <SecureStorage />;
       case 'saas': return <SaaSSettings />;
+      case 'billing': return <PricingPage />;
       case 'security': return (
         <div className="space-y-6">
           <div>