refactor: remove XML export, JSON-only for .rfflow files

- Remove XML builder, format query param, and XML tests
- Simplify ExportFlowModal (no format picker)
- Simplify rfflowParser (JSON-only)
- Remove format field from schemas and types

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/api/flowTransfer.ts

@@ -2,9 +2,8 @@ import apiClient from './client'
 import type { RFFlowFile, FlowImportResponse } from '@/types'
 
 export const flowTransferApi = {
-  async exportFlow(treeId: string, format: 'json' | 'xml' = 'json'): Promise<Blob> {
+  async exportFlow(treeId: string): Promise<Blob> {
     const response = await apiClient.get(`/trees/${treeId}/export`, {
-      params: { format },
       responseType: 'blob',
     })
     return response.data