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/layouts/AdminLayout.tsx

@@ -17,6 +17,7 @@ import {
   MoonOutlined,
   ApiOutlined,
   BookOutlined,
+  CrownOutlined,
 } from '@ant-design/icons'
 import { Avatar, Dropdown, Tooltip, Drawer } from 'antd'
 import { useAuthStore } from '@/stores/authStore'
@@ -44,6 +45,7 @@ const navItems: NavItem[] = [
   { path: '/usage', name: '用量统计', icon: <BarChartOutlined />, permission: 'admin:full', group: '运维' },
   { path: '/relay', name: '中转任务', icon: <SwapOutlined />, permission: 'relay:use', group: '运维' },
   { path: '/knowledge', name: '知识库', icon: <BookOutlined />, permission: 'knowledge:read', group: '资源管理' },
+  { path: '/billing', name: '计费管理', icon: <CrownOutlined />, permission: 'billing:read', group: '核心' },
   { path: '/logs', name: '操作日志', icon: <FileTextOutlined />, permission: 'admin:full', group: '运维' },
   { path: '/config', name: '系统配置', icon: <SettingOutlined />, permission: 'config:read', group: '系统' },
   { path: '/prompts', name: '提示词管理', icon: <MessageOutlined />, permission: 'prompt:read', group: '系统' },
@@ -207,6 +209,7 @@ const breadcrumbMap: Record<string, string> = {
   '/usage': '用量统计',
   '/relay': '中转任务',
   '/knowledge': '知识库',
+  '/billing': '计费管理',
   '/config': '系统配置',
   '/prompts': '提示词管理',
   '/logs': '操作日志',