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/pages/Workflow.tsx

@@ -0,0 +1,25 @@
+import { useState } from 'react';
+import { Tabs } from 'antd';
+import ProcessDefinitions from './workflow/ProcessDefinitions';
+import PendingTasks from './workflow/PendingTasks';
+import CompletedTasks from './workflow/CompletedTasks';
+import InstanceMonitor from './workflow/InstanceMonitor';
+
+export default function Workflow() {
+  const [activeKey, setActiveKey] = useState('definitions');
+
+  return (
+    <div style={{ padding: 24 }}>
+      <Tabs
+        activeKey={activeKey}
+        onChange={setActiveKey}
+        items={[
+          { key: 'definitions', label: '流程定义', children: <ProcessDefinitions /> },
+          { key: 'pending', label: '我的待办', children: <PendingTasks /> },
+          { key: 'completed', label: '我的已办', children: <CompletedTasks /> },
+          { key: 'instances', label: '流程监控', children: <InstanceMonitor /> },
+        ]}
+      />
+    </div>
+  );
+}