feat: add FlowCanvas (#593)

### What problem does this PR solve?

feat: handle operator drag
feat: add FlowCanvas
#592

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

web/src/pages/flow/index.tsx

@@ -0,0 +1,20 @@
+import { Layout } from 'antd';
+import FlowCanvas from './canvas';
+import Sider from './flow-sider';
+
+const { Content } = Layout;
+
+function RagFlow() {
+  return (
+    <Layout style={{ minHeight: '100vh' }}>
+      <Sider></Sider>
+      <Layout>
+        <Content style={{ margin: '0 16px' }}>
+          <FlowCanvas></FlowCanvas>
+        </Content>
+      </Layout>
+    </Layout>
+  );
+}
+
+export default RagFlow;