feat(workflow): add workflow engine module (Phase 4)

Implement complete workflow engine with BPMN subset support:

Backend (erp-workflow crate):
- Token-driven execution engine with exclusive/parallel gateway support
- BPMN parser with flow graph validation
- Expression evaluator for conditional branching
- Process definition CRUD with draft/publish lifecycle
- Process instance management (start, suspend, terminate)
- Task service (pending, complete, delegate)
- PostgreSQL advisory locks for concurrent safety
- 5 database tables: process_definitions, process_instances,
  tokens, tasks, process_variables
- 13 API endpoints with RBAC protection
- Timeout checker framework (placeholder)

Frontend:
- Workflow page with 4 tabs (definitions, pending, completed, monitor)
- React Flow visual process designer (@xyflow/react)
- Process viewer with active node highlighting
- 3 API client modules for workflow endpoints
- Sidebar menu integration

apps/web/src/layouts/MainLayout.tsx

@@ -8,6 +8,7 @@ import {
   SettingOutlined,
   MenuFoldOutlined,
   MenuUnfoldOutlined,
+  PartitionOutlined,
   LogoutOutlined,
 } from '@ant-design/icons';
 import { useNavigate } from 'react-router-dom';
@@ -21,6 +22,7 @@ const menuItems = [
   { key: '/users', icon: <UserOutlined />, label: '用户管理' },
   { key: '/roles', icon: <SafetyOutlined />, label: '权限管理' },
   { key: '/organizations', icon: <ApartmentOutlined />, label: '组织架构' },
+  { key: '/workflow', icon: <PartitionOutlined />, label: '工作流' },
   { key: '/settings', icon: <SettingOutlined />, label: '系统设置' },
 ];