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/lib/saas-client.ts

@@ -115,6 +115,18 @@ import { installRelayMethods } from './saas-relay';
 import { installPromptMethods } from './saas-prompt';
 import { installTelemetryMethods } from './saas-telemetry';
 import { installBillingMethods } from './saas-billing';
+export type { UsageIncrementResult } from './saas-billing';
+
+// Re-export billing types for convenience
+export type {
+  BillingPlan,
+  Subscription,
+  UsageQuota,
+  SubscriptionInfo,
+  CreatePaymentRequest,
+  PaymentResult,
+  PaymentStatus,
+} from './saas-types';
 
 // === Client Implementation ===
 
@@ -448,6 +460,7 @@ installRelayMethods(SaaSClient);
 installPromptMethods(SaaSClient);
 installTelemetryMethods(SaaSClient);
 installBillingMethods(SaaSClient);
+export { installBillingMethods };
 
 // === API Method Type Declarations ===
 // These methods are installed at runtime by installXxxMethods() in saas-*.ts.
@@ -506,6 +519,12 @@ export interface SaaSClient {
   // --- Billing (saas-billing.ts) ---
   incrementUsageDimension(dimension: string, count?: number): Promise<import('./saas-billing').UsageIncrementResult>;
   reportUsageFireAndForget(dimension: string, count?: number): void;
+  listPlans(): Promise<import('./saas-types').BillingPlan[]>;
+  getPlan(planId: string): Promise<import('./saas-types').BillingPlan>;
+  getSubscription(): Promise<import('./saas-types').SubscriptionInfo>;
+  getUsage(): Promise<import('./saas-types').UsageQuota>;
+  createPayment(data: import('./saas-types').CreatePaymentRequest): Promise<import('./saas-types').PaymentResult>;
+  getPaymentStatus(paymentId: string): Promise<import('./saas-types').PaymentStatus>;
 }
 
 // === Singleton ===