feat(admin-v2): add billing management page

- Plan cards with feature comparison and pricing
- Usage progress bars with quota visualization
- Alipay/WeChat Pay method selection modal
- Payment status polling with auto-refresh on success
- Navigation + route registration

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

admin-v2/src/router/index.tsx

@@ -27,6 +27,7 @@ export const router = createBrowserRouter([
       { path: 'agent-templates', lazy: () => import('@/pages/AgentTemplates').then((m) => ({ Component: m.default })) },
       { path: 'api-keys', lazy: () => import('@/pages/ModelServices').then((m) => ({ Component: m.default })) },
       { path: 'usage', lazy: () => import('@/pages/Usage').then((m) => ({ Component: m.default })) },
+      { path: 'billing', lazy: () => import('@/pages/Billing').then((m) => ({ Component: m.default })) },
       { path: 'relay', lazy: () => import('@/pages/Relay').then((m) => ({ Component: m.default })) },
       { path: 'knowledge', lazy: () => import('@/pages/Knowledge').then((m) => ({ Component: m.default })) },
       { path: 'config', lazy: () => import('@/pages/Config').then((m) => ({ Component: m.default })) },