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/CompletedTasks.tsx

@@ -0,0 +1,56 @@
+import { useEffect, useState } from 'react';
+import { Table, Tag } from 'antd';
+import type { ColumnsType } from 'antd/es/table';
+import { listCompletedTasks, type TaskInfo } from '../../api/workflowTasks';
+
+const outcomeLabels: Record<string, { color: string; text: string }> = {
+  approved: { color: 'green', text: '同意' },
+  rejected: { color: 'red', text: '拒绝' },
+  delegated: { color: 'blue', text: '已委派' },
+};
+
+export default function CompletedTasks() {
+  const [data, setData] = useState<TaskInfo[]>([]);
+  const [total, setTotal] = useState(0);
+  const [page, setPage] = useState(1);
+  const [loading, setLoading] = useState(false);
+
+  const fetch = async () => {
+    setLoading(true);
+    try {
+      const res = await listCompletedTasks(page, 20);
+      setData(res.data);
+      setTotal(res.total);
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  useEffect(() => { fetch(); }, [page]);
+
+  const columns: ColumnsType<TaskInfo> = [
+    { title: '任务名称', dataIndex: 'node_name', key: 'node_name' },
+    { title: '流程', dataIndex: 'definition_name', key: 'definition_name' },
+    { title: '业务键', dataIndex: 'business_key', key: 'business_key' },
+    {
+      title: '结果', dataIndex: 'outcome', key: 'outcome', width: 100,
+      render: (o: string) => {
+        const info = outcomeLabels[o] || { color: 'default', text: o };
+        return <Tag color={info.color}>{info.text}</Tag>;
+      },
+    },
+    { title: '完成时间', dataIndex: 'completed_at', key: 'completed_at', width: 180,
+      render: (v: string) => v ? new Date(v).toLocaleString() : '-',
+    },
+  ];
+
+  return (
+    <Table
+      rowKey="id"
+      columns={columns}
+      dataSource={data}
+      loading={loading}
+      pagination={{ current: page, total, pageSize: 20, onChange: setPage }}
+    />
+  );
+}