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/InstanceMonitor.tsx

@@ -0,0 +1,78 @@
+import { useEffect, useState } from 'react';
+import { Button, message, Space, Table, Tag } from 'antd';
+import type { ColumnsType } from 'antd/es/table';
+import {
+  listInstances,
+  terminateInstance,
+  type ProcessInstanceInfo,
+} from '../../api/workflowInstances';
+
+const statusColors: Record<string, string> = {
+  running: 'processing',
+  suspended: 'warning',
+  completed: 'green',
+  terminated: 'red',
+};
+
+export default function InstanceMonitor() {
+  const [data, setData] = useState<ProcessInstanceInfo[]>([]);
+  const [total, setTotal] = useState(0);
+  const [page, setPage] = useState(1);
+  const [loading, setLoading] = useState(false);
+
+  const fetch = async () => {
+    setLoading(true);
+    try {
+      const res = await listInstances(page, 20);
+      setData(res.data);
+      setTotal(res.total);
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  useEffect(() => { fetch(); }, [page]);
+
+  const handleTerminate = async (id: string) => {
+    try {
+      await terminateInstance(id);
+      message.success('已终止');
+      fetch();
+    } catch {
+      message.error('操作失败');
+    }
+  };
+
+  const columns: ColumnsType<ProcessInstanceInfo> = [
+    { title: '流程', dataIndex: 'definition_name', key: 'definition_name' },
+    { title: '业务键', dataIndex: 'business_key', key: 'business_key' },
+    {
+      title: '状态', dataIndex: 'status', key: 'status', width: 100,
+      render: (s: string) => <Tag color={statusColors[s]}>{s}</Tag>,
+    },
+    { title: '当前节点', key: 'current_nodes', width: 150,
+      render: (_, record) => record.active_tokens.map(t => t.node_id).join(', ') || '-',
+    },
+    { title: '发起时间', dataIndex: 'started_at', key: 'started_at', width: 180,
+      render: (v: string) => new Date(v).toLocaleString(),
+    },
+    {
+      title: '操作', key: 'action', width: 100,
+      render: (_, record) => (
+        record.status === 'running' ? (
+          <Button size="small" danger onClick={() => handleTerminate(record.id)}>终止</Button>
+        ) : null
+      ),
+    },
+  ];
+
+  return (
+    <Table
+      rowKey="id"
+      columns={columns}
+      dataSource={data}
+      loading={loading}
+      pagination={{ current: page, total, pageSize: 20, onChange: setPage }}
+    />
+  );
+}