feat: add subscription UI, plan-based nav gating, and client subscription page

- Add plan field to UserInfo shared type
- Subscription API client and React Query hooks
- Client subscription page with status + payment history
- Sidebar navigation filtered by tenant plan features
- Subscription link added to navigation

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

packages/shared/src/types/auth.ts

@@ -29,6 +29,7 @@ export interface UserInfo {
   tenantId: string;
   tenantName: string;
   tenantRfc: string;
+  plan: string;
 }
 
 export interface JWTPayload {