Add BPMN import/export functionality to workflows:

- Implement `handleImportBpmn` and `handleExportBpmn` methods in `WorkflowsPage` for managing BPMN files.
- Add file input and buttons for importing/exporting BPMN diagrams in the UI.
- Extend `workflowService` with `importFromBpmn` and `exportToBpmn` methods.
- Update `httpClient` to handle non-JSON responses.