feat: add flow export/import frontend + backend tests

Frontend:
- ExportFlowModal with JSON/XML format selection + download
- ImportFlowModal with drag-drop file picker + preview step
- rfflowParser for client-side JSON/XML .rfflow parsing
- Export buttons on editor toolbar and library action menus
- Import button on library page next to Create New
- Provenance display for imported flows in editor
- flowTransfer API client + types

Backend:
- Fix regex->pattern deprecation in export endpoint
- 12 integration tests covering export, import, round-trip,
  access control, tag/category creation, version validation

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

frontend/src/api/index.ts

@@ -20,3 +20,4 @@ export { default as aiBuilderApi } from './aiBuilder'
 export { default as aiChatApi } from './aiChat'
 export { copilotApi } from './copilot'
 export { assistantChatApi } from './assistantChat'
+export { flowTransferApi } from './flowTransfer'