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/ProcessDefinitions.tsx

@@ -0,0 +1,122 @@
+import { useEffect, useState } from 'react';
+import { Button, message, Modal, Space, Table, Tag } from 'antd';
+import type { ColumnsType } from 'antd/es/table';
+import {
+  listProcessDefinitions,
+  createProcessDefinition,
+  publishProcessDefinition,
+  type ProcessDefinitionInfo,
+  type CreateProcessDefinitionRequest,
+} from '../../api/workflowDefinitions';
+import ProcessDesigner from './ProcessDesigner';
+
+const statusColors: Record<string, string> = {
+  draft: 'default',
+  published: 'green',
+  deprecated: 'red',
+};
+
+export default function ProcessDefinitions() {
+  const [data, setData] = useState<ProcessDefinitionInfo[]>([]);
+  const [total, setTotal] = useState(0);
+  const [page, setPage] = useState(1);
+  const [loading, setLoading] = useState(false);
+  const [designerOpen, setDesignerOpen] = useState(false);
+  const [editingId, setEditingId] = useState<string | null>(null);
+
+  const fetch = async () => {
+    setLoading(true);
+    try {
+      const res = await listProcessDefinitions(page, 20);
+      setData(res.data);
+      setTotal(res.total);
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  useEffect(() => { fetch(); }, [page]);
+
+  const handleCreate = () => {
+    setEditingId(null);
+    setDesignerOpen(true);
+  };
+
+  const handleEdit = (id: string) => {
+    setEditingId(id);
+    setDesignerOpen(true);
+  };
+
+  const handlePublish = async (id: string) => {
+    try {
+      await publishProcessDefinition(id);
+      message.success('发布成功');
+      fetch();
+    } catch {
+      message.error('发布失败');
+    }
+  };
+
+  const handleSave = async (req: CreateProcessDefinitionRequest) => {
+    try {
+      await createProcessDefinition(req);
+      message.success('创建成功');
+      setDesignerOpen(false);
+      fetch();
+    } catch {
+      message.error('创建失败');
+    }
+  };
+
+  const columns: ColumnsType<ProcessDefinitionInfo> = [
+    { title: '名称', dataIndex: 'name', key: 'name' },
+    { title: '编码', dataIndex: 'key', key: 'key' },
+    { title: '版本', dataIndex: 'version', key: 'version', width: 80 },
+    { title: '分类', dataIndex: 'category', key: 'category', width: 120 },
+    {
+      title: '状态', dataIndex: 'status', key: 'status', width: 100,
+      render: (s: string) => <Tag color={statusColors[s]}>{s}</Tag>,
+    },
+    {
+      title: '操作', key: 'action', width: 200,
+      render: (_, record) => (
+        <Space>
+          {record.status === 'draft' && (
+            <>
+              <Button size="small" onClick={() => handleEdit(record.id)}>编辑</Button>
+              <Button size="small" type="primary" onClick={() => handlePublish(record.id)}>发布</Button>
+            </>
+          )}
+        </Space>
+      ),
+    },
+  ];
+
+  return (
+    <>
+      <div style={{ marginBottom: 16 }}>
+        <Button type="primary" onClick={handleCreate}>新建流程</Button>
+      </div>
+      <Table
+        rowKey="id"
+        columns={columns}
+        dataSource={data}
+        loading={loading}
+        pagination={{ current: page, total, pageSize: 20, onChange: setPage }}
+      />
+      <Modal
+        title={editingId ? '编辑流程' : '新建流程'}
+        open={designerOpen}
+        onCancel={() => setDesignerOpen(false)}
+        footer={null}
+        width={1200}
+        destroyOnClose
+      >
+        <ProcessDesigner
+          definitionId={editingId}
+          onSave={handleSave}
+        />
+      </Modal>
+    </>
+  );
+}